You have to evaluate them on an individual basis.
condition is important too...
Both true. Be careful of the "it's just a dollar" mentality. You can easily wind up with hundreds of dollars tied up in wrinkled cash when they're worth close to face and are being eroded by inflation. Sometimes you might get lucky and find a star with a low print run, you just have to (as Steve said) evaluate each one.
no kissing, please!