Of our windows, only 5 of 13 open at this point. The upstairs has one size, the downstairs has a slightly larger size and the kitchen window is an over-the-sink one. Is it necessary to measure the 6 distances (3 x width, 3xh) on each
window or would measuring one
window of each size be sufficient? The current windows are wooden, 1955, single pane, with spiral tube balances, and not window weights. They are original to the house.
You are probably all right if they are wood pocket windows and you are going to be installing insert replacements in the exisisting frames after removing the parting bead/trim stop and sashes. Odds are that if one window where that far out of square that you would see issues where the sashes and frames meet. Metal framed windows typically require more care in measuring because they can be non-standard sizes. Most wood windows are standard sizes and given that you are deducting a 1/4 inch from the width and height, you should be fine.
That all being said, we always measure every window so take the extra time and measure them all.