Saliva is too much work, and I honestly don't trust it to be enough. I use lube. LovRub usually. It's pleasure enhancing and all that jazz so it is a lot funner, and hurts less for her.
That would be up to the lady to answer. I have had GF's that prefer a little friction, so they like spit for entry. But mostly its Rule 1 - Too much Lube is almost enough.
My wife recently asked me to stop using lube for anal sex as she does not like the feel of the lube on her skin. She just likes me to use spit now. We don't use condoms and I am un circumcised so not as much drag as a circumcised guy would get. Anyway, its cool as I don't like it much when her pussy tastes of lube, that stuff gets everywhere.
Definitely use lube. Saliva is not a sufficient lubricant. By using lube you will increase your chances of being able to enjoy anal play again...