What effects does sucking on a pacifier have on teething?

Depends. Apparently, it can increase the risk for "buck " teeth, or teeth that are malaligned.
During teething. During teething it is actually likely that they will chew on the pacifier and it will actually help cut the teeth, so no prob. But know too that pacifiers can cause bone malformations and tooth alignment problems if used too long, beyond age 5 especially.