I do the same as you do. The convenience of doing parameter validation early on and bailing out early on is IMO that it binds parameter validation visually to the function (if that makes sense). When you're skimming over the function while reading the code, it's easy to see what parameters are expected and what their format and restrictions are.

Also, I find that it helps me focus when I write a function: first, validate parameters, then worry about the algorithm / implementation / whatever without having to think about invalid parms.

So, for the sake of self-documenting code, I'd put param validation at the top of the function. The added exit area is not so much of a problem, as you're still only exiting at the start or end, not in the guts of the routine.

CU
Robartes-