Is this question about vibrato (i.e., making the voice quaver a bit on the ends of notes; the setting adjusted by clicking the box to the lower right of the note)? If so, you might get more feedback if you change the question from "tremolo" to "vibrato."
In the past, I've usually let Piapro add vibrato if it wants to, since it will only add it to notes that are long enough by default, as I haven't generally thought it made the sound bad. It's easy enough to remove if you want to, as I've done on occasion--when you open the vibrato settings for a given note, there's an option for "none," I believe. You could always turn that around, though, too--turn it off be default and then turn it on for individual notes. It's completely up to you and what kind of style you want to use. I imagine the off-by-default approach is probably more similar to how a human singer would behave...except that I also assume it would be hard for a person to keep a sustained perfectly steady pitch, depending on the note length, which may be why Piapro adds vibrato by default.
Personally, I would opt for curves over straight lines, just under the assumption that organic things tend to be curved rather than having nice straight geometries. There's a curve tool in Piapro that should help with making curves.
I personally think that there's sort of a progression for how to do vibrato using the methods you've discussed, depending on how much you want the pitch to fluctuate. For general use/smaller warbles, I'd use the default vibrato controls. If those don't vary the pitch enough/in the way you need, use the pitch bends instead (Don't forget to set the pitch bend sensitivity as you require, to control the available range of the fluctuations.). Finally, if you need something really extreme (Or if you're not particularly skilled at drawing pitch bends, maybe? It seems to me like trying to draw vibrato with pitch bends by hand could be hard.), use the DAW plugin. IMO, that should maybe be a last resort, though--syncing up the automation in your DAW to the timing of the notes would be a small pain.
