I agree with the other commenters that it's a stylistic decision. I just want to mention a great way to transition from tripod to handheld. I can't remember what movie it was in; might have been James Cameron action flick. (Love his camera work!) The camera is static or moving smoothly, until there is an explosion that appears to jostle the camera. The next shot is handheld, as are subsequent shots.
I've seen many movies where the camera work, and the editing, are designed to match what a viewer's eyes would do. They dart to interesting elements in the scene; they look at the person who is talking, or who has the most important emotion or reaction in a conversation. If you use a style like that, you could start increasing the pace of the cuts right before you move to the handheld shots. And to "come out of" the handheld section, you can slow down the shakiness, and use a character's emotional transition from erratic to calm to motivate the camera's transition from handheld to static.
As I've been writing this, I realized that the camerawork (editing pace, tripod vs. dolly vs. handheld, etc.) can match the emotions of a character or the emotions of the scene, if you want. That will make the different camera styles "feel" right to the audience. But just like you don't want your actors to overact, you don't want your camerawork to be overdone either.