My pinwheel blocks are made of 1.5" HST, meaning they are a 2.5" unit, finished size 2". The centre part of the block is 6" finished; 5 pinwheel blocks and four 2.5"(cut size) squares. The large triangles are made from 5.25" squares, cut once diagonally. the final block measures 9", 8.5"finished size.
I press all the seams of the pinwheels open, to distribute the bulk of the seam allowances better. The rest of the block is pressed the conventional way, seams to one side.

I pin the little HSTs together at every stage too, because I'm in the mood to be particular with this quilt.

I like seeing that all the seams meet exactly; if they don't I just shrug and accept it, but I try for perfect every time. That's what all the pins are for.
When it comes time for that last seam I line up the points that I want to meet and place a pin through them and jockey them into position; then I use another pin to hold them in that place while I sew. The vertical pin is removed as soon as that other one is in position.

I found my top intersections were consisently pushed past the bottom, so now I lean that first pin slightly to the left before placing that final one and the finished block is more accurate.
And because I have terminal distractability I am now making two leader ender projects while I sew the pinwheels. One is all my random 2.5" HSTs that have been building up for a while. I just want to make a heap of 8" blocks and use them with my orphans or something similar. The cross block is part of a huge stack of pieces I cut out in some mad spree at the start of the year. I must have been hallucinating, because I have no idea what half the pieces are for. I'm just sewing together what fits, and I'll puzzle over what's left. Maybe I'm at that stage of life where I need to write myself notes for the future......