My son is 5 months and he puts his thumbs in his mouth and presses up against his gums (teething) but never actually sucks his thumb. Sometimes I see both thumbs in there pressed up against the top of his gums, one on each side.
A pacifier has never really entered my thoughts, so he doesn't have one.
I prefer paci to thumb, if they take one at all. The paci you can throw away when they are too old for it but the thumbs you can't get rid of. It's a harder habit to break when they are thumb suckers. I have never had a thumb sucker but I never encouraged or discouraged it either. None of them ever even really tried to suck their thumbs.
Neither one of the twins suck their thumbs or take a paci either. As long as they are fed and their diapers are dry they are happy.
Only Jacob and Isaac ever took a paci while babies. Clara found one in a toy box when she was about 14 months old and decided she liked it and we took it away at 2 yrs old. I don't let them have it past 2 yrs.
Rachel Mommy to DS13, DS12, DS7, DD5, DD3, and twin GIRLS : born at home in the water on 12/18/09