I personally like using a variety of lenses for capturing portraits here is a shot on my Canon 30D and Sigma 120-300mm f2.8 at 300mm and f2.8 which allows for great candids but would not be my choice for general portrait work, for that I like using my Canon 5D and Sigma 70-200mm f2.8 or Canon 24-105mm f4 if I need to pull back a bit. As I'm lazy I don't often use the 50mm f1.8 or 85mm f1.8 primes (although that latter is fantastic) I'm not keen on the whole zoom with feet LOL (I'm sure if would be good for me if I did though..... I need some exercise!!).
If we are being purists then 90mm and just over in 35mm terms is the 'ideal' so with a 1.5x crop digital then a 60mm lens would fall nicely into this. Themain reason this length is the norm is that then you don't get the nasty distortions of close and wide. Here is when notto use 24mm on a Canon 5D.... now I'm not saying you can't use wide lenses but it is knowing when and how, likewise with longer than normal lenses.
Attached shot with Canon 5D and Sigma 70-200mm f2.8 at 70mm f13.