When possible, I never bounce soft synth tracks. That's just OCD, though. The thought that I might want to make a last-minute tweak keeps me from commitment.
In the case of drum tracks, I do make changes right up to the end because they're fake drums I'm trying to make sound more realistic. Real drummers interact with and support the other instruments. You don't want a metronome drone for a drum track, even if that's what you initially started out with. I add ghost notes, accents, advances and short fills in places where the mix needs them, and can't know where those places are until late in the mix.
There is a downside to this, however. One obvious problem is you might exhaust your RAM and CPU and have no choice but to freeze some synths. But I ran into a less-foreseeable problem when I moved from a 32-bit system to a 64-bit system and there were a couple synths and effects that weren't compatible. I lost the ability to modify a few projects as a result. Fortunately, my old computer had been underpowered so most tracks had been frozen out of necessity. Losing synths and effects is a real possibility due to draconian licensing schemes that constantly threaten to disable themselves, so freezing them is insurance against that.
Another possibility is that a synth or sample library doesn't sound the same after freezing. It's not common but it does happen. There have been times when I didn't realize the problem, until I un-froze a synth for editing only to find that I could not get back to its previous sound. Sample libraries can be prone to this, if you've installed an update or modified an .nki since the original library was frozen.
So to summarize: freezing optimizes computer resources and can be good insurance. But it does restrict your creative freedom. Security versus freedom, it's an age-old balancing act.