I have never used a light meter and am only slowly getting to grips with off camera and manual flash but here are my thoughts....
If you are shooting outdoors on a bright sunny day the first thing to think about is getting control over your ambient light. If you are using flash then that puts a particular constraint on you due to the maximum sync speed - normally 1/250, but only 1/200 on the Rebels. If we think about good old Sunny 16.... If you are at 100 ISO and shooting at 1/250 you will need an aperture of f/10 for a "correct" ambient exposure. If you shoot any slower than 1/250 - say 1/200 then you will need to go to f/11 and so on, up to 1/100 and f/16, for example.
But f/16 is probably not the best choice for portraits unless you want to see the whole backgrond in focus too. Also, f/16 is going to make your flash work very hard in order to have much impact, so you'll likely be hard up against that 1/250 wall, in order to open up your aperture a bit. If you do want a wider aperture for creative DOF you can always throw an ND filter on the lens, which is great for the background, but your flash will also need to battle its way through the filter so it will not give you any advantage in managing the lighting balance. It will give you the magic bokey though, with a decent lens.
So you need to figure out what your creative goals are for aperture and DOF, and whether you want the background brighter or dimmer than a "standard" exposure. Then you can work out what else you need on top of that in terms of adding flash and how all that influences your ambient exposure settings. If your subject is in the same light as the background and you want to add flash as well then you will probably need to set the ambient exposure a little below "optimum" because you need to leave some room in the exposure settings for the extra light from the flash, without overexposing your subject. i.e. you will want to underexpose your background slightly. The alternative is to shade your subject with a large diffuser panel, or just position them in a shadey spot (open shade or even shade, not dappled) and then light as you wish with the flash. This will create far less dependence between the ambient and flash exposures. Check out some of this guy's work....
http://www.youtube.com/watch?v=sqMI72jsXRQ&
http://www.youtube.com/watch?v=brSWJZmJ2m8
http://www.youtube.com/watch?v=g18tXTmm-Qs
If the ambient light is less than bright sunshine then your job gets easier, as your 1/250 limit becomes less of a constraint, and the weaker ambient light becomes easier to dominate/overcome with flash power. Let's assume the ambient light is such that you can work at 100 ISO, f/8 and 1/100, so the ambient is a couple of stops dimmer than Sunny 16 conditions. If you had a single 580EX flash gun your maximum working distance, with a 100mm lens, would be 58 (guide number) / 8 (f/stop) = 7.25m or about 23'. With a wide lens it would be less. Now, lets say you can place the flash gun half that distance from your subject - 3.6m or 11.5' away. Here's where the fun begins....
You can completely control your background/ambient exposure by adjusting shutter speed. If you get a "correct" exposure at 1/100 you can dim the background by 1 stop by going to 1/200 and there will be no effect on your flash exposure. If you up the shutter speed to 1/250 then you can get your background underexposed by 1 1/3 stops. If you want a really bright, perhaps blown out, background then you can slow the shutter below 1/100. 1/50 will give you a background that's nominally 1 stop overexposed. If your subject is in the same light then it will become overexposed to, so you have to watch how you work this a bit. So basically you can jiggle your background/ambient exposure up and down just by varying shutter speed, right up to the sync speed limit. Changing shutter speed, up to your maximum sync speed, has no effect on the flash exposure at all, because the flash duration is so short (it has to be shorter than the sync speed) that a longer exposure will not gather any extra flash light, because there isn't any more flash light to gather. That was all done after the first 1/1000 of a second or so.
With complete control over the background lighting established, you can now play around with your flash exposure. Remember we positioned the flash at half maximum distance. Well that means we can nominally run the flash at 1/4 power (inverse square law at work). If you move the flash closer or further you will affect the light quality, so that's not a good technique. But you now have plenty of flash power in reserve. So if you want to brighten your subject just turn up the flash power in manual or the FEC in ETTL mode. If you want to reduce your subject brightness then just dial the flash down a bit.
You could, if you wish, vary the flash exposure simply by altering the aperture, but that would also affect the exposure for the entire scene (and also your DOF), so to keep the background exposed correctly, for every stop you open up the aperture you need to speed the shutter up by a stop - until you hit the sync speed brick wall.
In a studio, with relatively low ambient light, the ambient light basically has no material effect on the picture, and everything is accomplished by positioning and controlling your strobes. You use the meter to establish the relative balance between the lights and then get a final meter reading to see what f/stop you should set the camera to for the whole lighting package. Shutter speed basically has no effect, because all the light the camera sees is delivered in a couple of milliseconds (or less) by the strobes, so you can shoot at 1/125, 1/160, 1/200, 1/250 and it really should make no difference to the exposure. Varying your aperture does.
