I use WTM, WEM, Invitation to the Classics, and Veritas Press catalog :) (basically what would be used in Omnibus). If I get a consensus I generally go with that.
Just a thought - it might not be worth anything - but I got most of the ancients and medieval books from buy.com, they're about $1-2 less than Amazon - if you're getting a lot of books, that can add up. Plus they used to give $10 off every Google checkout - I did that a LOT. I'm not sure whether they still do that, but I saved a lot that way. :)