I nearly lost a whole section of fence last month because the clay was too soft. I waited a day for it to dry out a bit, then packed gravel around the bases. The posts are rock solid now, and it saved me from having to redo the work. Always check the ground conditions first.
Man, i used to swear by setting posts wet and just bracing them tight. Then i saw a whole line of fence posts crack when the concrete cured too fast in summer heat. @jordan381, watching them all lean the next spring even with good braces changed my mind completely. Now i always pack dry mix around the post and let the ground moisture set it slow, no cracks and they stay put.
Listen to anthonyy55, he's seen the concrete crack and the posts lean. Wet setting is like trying to rush a cake, it just falls apart. Trust the guy who already watched his fence do the wave.