It depends on the editor, sometimes it helps but only if the editor is in a good mood! Other times I imagine it can put you at teh back ofteh queue. If you do, be VERY polite!
After waiting more than 6 months, I actually got mine listed when I did a polite e-mail to the catagory head editor at DMOZ. By chance the editor had been removed and no one was reviewing my catagory. You never know, if its been more than six months it doesnt hurt to ask.