I don't cut larger, but I do check units for size and trim if needed. I use a scant 1/4" seam, and prefer having something to trim to having a unit that is shy of size. I also need a narrower seam when triangles are involved, as bearisgray mentioned.
Regarding the Peaky and Spike issue, I could not get those units right when using the Tri-Recs rulers. I think the Deb Tucker V-block is a good option (although with that, my seam allowance at the bottom seems to be slightly narrow). Last year I had to start using an Accuquilt cutter so as to lessen the amount of rotary cutting I did, and I found I could get a good V-block with the Accuquilt if I used a scant seam allowance and pressed with extreme care.