I still consider myself a beginner in both, creating and playing MP levels so pardon me if I'm totally wrong but I could just never understand the idea of "putting ammo where it's the most useful". So many level creators seem proud saying their level is like that, while I wouldn't really think of it as of an advantage. Contrary, I believe a much better idea is to place it where it's totally useless - discourages camping and keeps a player in move, because he/she has to collect ammo and use it in different places. This applies to powerups too, and not only this - imo it's also a good idea to place powerups of only the most useless (in a given level) weapons. This way using ammo that isn't powered up becomes an actual alternative: if it's easier to hit an enemy with it, who cares it deals lower damage?

Btw, what I said is not advice, rather another question. I didn't try these ideas in a level yet. I'd like to hear a professional opinion on my theory, and are my ideals all wrong or could I base a separate style on them, leading to good levels.