"I have gone outside in the mornings following my spring repottings and found everything frosty because it dipped to about 28-29, and haven't lost a leaf."
Me too, but I've also discovered frost damage on leaves, shoots and sometimes trunks as growth progressed. I've also had smaller trees that don't have much soil to protect them frozen solid.
Late freezes, when trees are on the cusp of growth, are very tricky to handle. The bonsai two step is a necessity in my area when cold lingers a little longer, or when winters are hard (like this one). My dance involves several trees that weigh 100 lbs or more. It's not fun, but better safe than sorry.
I stopped depending on the local weather people to reliably predict nighttime lows some time ago. I've found there is a five to even ten degree discrepency in what they predict and what actually happens in my backyard. When trees are starting to push growth here, that kind of gap can be fatal for them. Whenever the local weather people predict low temps in the mid-30's in March, I move trees that have pushed growth inside. Sometimes they wind up staying inside for a couple of days, which doesn't hurt them.