A 50mm lens (macro or otherwise with ext. tubes) puts you awfully close to your subject at 1:1 magnification.
In theory, you need extension tube length equal to the focal length of the lens to get to 1:1. So a 50mm lens should need 50mm of extension behind it. That's a pair of Canon 25mm tubes stacked, or a Kenko 36mm and 20mm (approx.).
In reality, the simple formula of how much extension tube to use doesn't take into account the lens' inherent close focusing capabilities. Plus, what many people shoot and think of as macro is probably closer to 1:2. So, often you don't need that much extension behind the lens. Still a 50mm lens can put you pretty darned close to the subject.
Personally I prefer a somewhat longer macro lens for field work, just to be a little farther from the subject. Too close and you might cast a shadow on it or bump it, or scare away living critters, or get bit or stung! Most of the time I use 90mm or 100mm (Tamron and Canon 100/2.8 USM, non-I.S.).
There are times when a shorter focal length macro is useful. For indoor, studio tabletop shooting such as small products, I usually use a 45mm Tilt-Shift lens. In this case I want the shorter focal length so that I can reach out and arrange the subject while keeping my eye to the viewfinder. It's all tripod work and studio lights and reflectors pretty much eliminate concerns about casting a shaddow on the subject.... plus there's no danger of getting bitten or stung!
At the other extreme, for anyone shooting with a crop sensor camera, I think a 150mm or 180mm might be a bit too long a macro, for general purpose field work. With longer lenses you end up with shallower depth of field, which means you have to stop down a lot more, which risks starting diffraction (loss of image quality) and forces you to use slower shutter speeds, while trying to keep a longer lens steady. It's just more difficult than using a lens in the 60mm to 105mm focal lengths. There are a lot to choose from that all seem to work quite well:
Canon EF-S 60mm f2.8
Sigma 70mm f2.8
Tamron 90mm f2.8
Canon EF 100mm f2.8 USM (non-I.S., now discontinued but still widely available)
Canon EF 100mm f2.8L I.S.
Tokina 100mm f2.8
Sigma 105mm f2.8
Even with a bunch of macro lenses (four in my Canon kit, others in vintage kits I still use some times)... I always carry a few extension tubes and have always done so with various camera systems over the past 25 years. They are just so handy, can be used with practically any lens. I use the Kenko set, two Canon 12mm and one Canon 25mm. I've used them on lenses from 20mm to 500mm. Sometimes it's just to improve a lens' minimum focus distance. Other times it's to get into more truly macro work, such as increasing a macro lens' mag to well beyond 1:1. (I don't have the MP-E 65mm which is a specialized macro lens that starts at 1:1 and goes to 5:1 magnification.)
