Waves Trans-X likes a slow bounce. Many Omnisphere patches prefer it. Sometimes very large Kontakt libraries need it. If you're using an external hard drive for samples the probability goes up that you'll need a slow bounce. Exhausting RAM may prevent a fast bounce.
As a general rule, you'll avoid problems with the slow bounce. I have only encountered one instance where a plugin required a fast bounce: using MIDI automation with SampleTank. In one project, I had both MIDI in ST2.5 and Trans-X. To work around the conflicting requirements, I first froze SampleTank with a fast bounce and then exported the project using slow bounce.
But these are all cases where particular plugins just don't work right with a fast bounce, and have nothing to do with audio quality. 1 + 1 = 2 whether you do it as fast as possible or wait 22.7 microseconds at the equals sign.