Oh heck yeah I mix and match all the time...if I had a wallet to match each bag I'd be extremely poor, ha ha.
I have a mono zippy wallet which I love and she goes in all my LV bags...and even in my Coach Signature and Fendi Zucca! OK, that's a little hard on the eyes sometimes, but at least the wallet isn't out on display all day.
Well, since I never change wallets (I have a Mono Koala) and I change bags almost every day. I have Mono, Damier, Mini Lin and a variety of Gucci bags. So sometimes I match and sometimes I don't. I don't think it matters either way.
mixing and matching is the best i think. it keeps the people guessing (esp folks who work at the lv stores) when you pull out certain wallets, agendas, cles, pochettes that are all different. theres' not rule at all imho.