There's a lot that goes into that answer. Basically it comes down to what part of our coffee lineup we're trying to fill. You can do any coffee any way you want - there's no wrong or right. That's one of the beauties of coffee. I can take one coffee and roast it, and another roaster can take the same exact coffee and come up with something completely different.
In general (and I mean VERY VERY GENERAL.... there are no rules here), I tend to roast African coffees on the lighter side, Central American coffees tend to be more medium roast coffees and Indonesian coffees are sometimes a bit darker. However, I also have a dark roast Ethiopian Yirgacheffe, and I've done a Papua New Guinea on the medium/light side. So it's all a bit weird.