QuiltE ... I use what ever method suits me at the time, or the fabric.
I've tried using the entire FQ then just trimming the parts I didn't need. I didn't like that approach as there was far too much fabric to handle and it was easy to sew parts of the excess that I didn't want sewn!! Nor does it allow you to "chain piece" when you have multiple duplicate blocks (like this project - which I am chain piecing).
One of the "spikes" that I'm making now (the first block) I'm using templates for the center bit of the flower only because I want to fussy cut the fabric that I'm using to get the medallion centered in the middle of the flower.
I might also use templates if I need to make multiples of the same piece ... triangles for example that when flipped top to bottom will fit neatly across a strip of fabric. But I only use them as a general guideline.
MOST of the time, I use a general guestimate as to the size I need, cut it larger then trim it down. That said ... when I'm usuing prints I OFTEN forget to flip the fabric and the fabric I cut is a mirror of what it should be!! Grrrr. Which is why I *prefer* to use either batiks or solids when PP ... makes life soooo much easier.
Unfortunatley for this project I didn't have enough batiks or solids in the color scheme I wanted. So I'm trying to be VERY careful!!