Doing these mock drafts on Yahoo is a great idea as I myself have already done one. But there should be a disclaimer that once the draft is over you simply can't just delete your team, nor can the commish delete the league. That league is up for the rest of the season. Yahoo allows a user to join up to 4 leagues per Yahoo ID. Joining one of these will take up a spot where you could join a real league. If you do these I recommend you create a "dummy" Yahoo ID so you don't waste league spots on your regular Yahoo ID.
I'm not trying to dissuade anyone. i just wanted to make people aware of the circumstances. This is a great opportunity to see what everyone else is drafting, plus if you're a newbie of Yahoo it gives you a chance to practice with their live draft format so you're more comfortable with it when your real draft happens.