Since I do a lot of hand applique, I cut them larger & then trim down to size because sometimes the applique will slightly distort the fabric & then I'm off by 1/8" somewhere. Much better to cut larger & trim down, imho.
For piecing without applique, I cut to the exact size. I have a triangle ruler & it has been a godsend. Haven't had any issues with my triangles since I bought it.