Bouncing flash in a church?

Since I just purchased the 580EX I still need to learn lots about how to use it.
During the ceremony inside the church do you rather bounce the flash on the (usually very high) church ceiling or do you point the flash straight on. I am thinking that straight on will cast to harsh of a shadow?

Also, forgive my ignorance, but can I set this flash to a lower setiing somehow so that I can use it only to fill in my otherwise no flash pictures? Do you know what I mean...

Sometimes when I shoot no flash and I take the widest aperture and slowest shutter I can do without shake (1/60th) it is still a bit to dark. But adding a full power flash would be too much. I want to preserve the look of a no flash pic by just adding a bit extra light. Is that possible with this flash?

You can set the FEC on the camera body or the flash (flash exposure compensation), that should do the job. Thing about fill is it's not going to help slow shutter speeds, it just fills in shadows. If you have it firing out enough light that it's the main light it's going to be distracting. I don't use flash in ceremonies, I use an F2.8 lens, if that's not enough I use the 50mm F1.4 and stand closer. Actually I did use fill flash once, but that's only because I forgot to turn the flash off.

Bouncing in a church isn't practical either.

I'll offer the opposing viewpoint (so you can see that there are a few "right" ways). I almost always use a flash in the church - at least for the processional, recessional, and kiss - more if needed (and allowed). In a church, I am typically using my flash as the main light - not for fill (i.e. if I have enough light that I only need a little fill I'd rather turn the flash off).

Whether I bounce or not is a decision I make on a case by case basis, just like I would if I weren't in a church. Are the ceilings high enough/too high? Are they white? For cathedrals with very high, dark ceilings, I point my flash straight ahead, and try to balance the flash/ambient with a combination of high ISO/slowish SS.

The reason that I use a flash during the processional/recession​al is that the aisle of most churches is not as well lit as the altar (where I can get away with natural light). For most of the processionals, the back door of the church is open making me deal with a strong backlight. The flash fills the front.

The reason I use a flash for the kiss, is that I am usually at relatively slow shutter speeds during most of the ceremony when I am shooting available light. The kiss happens so fast, and is such an important moment that I don't want to risk an unusable shot due to my/the couple's movements.

As for FEC - to me it is the most important button on my camera when my flash is the primary light source. I ride it all day, making adjustments regularly based on what my histogram looks like.

very interesting points Tim and Jamie, thanks

I do a lot of what Jamie does, using flash for the processional/recession​al, but usually not for the kiss.