As others have said - try it both ways and see which works out better for you.
I have cut and sewn many triangles that I cut using a template - which makes the triangle a "wee bit" bigger than if I cut a (finished size + 7/8 inch square) into triangles - and they turn out close to "expected" size.
Even when sewing what I consider "very scant" 1/4 inch seams with the (finished size + 7/8 square) - I almost always end up with skimpy units.
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