It's better to use the seam width that will give you the expected size of the blocks/units you are making.
A good test - or trial run - for any project:
Cut three strips of the fabrics you plan to use 'your' two inches wide by 5 inches long
Sew them together with 'your' 1/4 inch seam and the thread you plan to use
Press the seams the way you press them - to the side, open, whatever
Measure the unit you just made - it 'should' measure 5 inches by 5 inches - and the center strip should be 1-1/2 inches wide.
If it's smaller than expected, make narrower seams.
If it's larger than expected, make wider/deeper seams.
Some variables that can affect the finished size:
Were the fabrics washed/shrunk before cutting - some shrink when steam-pressed
Cut size of pieces
Seam width
Thread thickness
Pressing technique
When I say 'your' measurements of the cut pieces - unless one is using a die cutter, one can have small size variations - depending on which ruler one uses, where one places the ruler on the fabric, etc.