If you are making HST out of the strips, my best guess is that regarless if they are 3 3/8 or 3.5", you will be sewing two together and squaring up the finished unit to 3". So I do not think that the size of the border will be affected.
I make alot of HST's, and always cut up to the next 1/8 inch. (The measurements for HST's are either at 3/8 or 7/8, which means to me, 1/2 or next full inch)