I've used graph paper that has the 1/4 inch grid to help me figure things out.
I line up the right edge of the presser foot with a line on the graph paper. Then I "sew" a few stitches to see where the stitching line is.
Then, depending on the machine, I can adjust the needle position, or I can mark where the edge of the fabric should go with tape, post-it notes, or whatever.
Some machines have lines marked by the feed dogs that are helpful - the graph paper can help "calibrate" these lines with the presser foot.
The "sample test" using several strips of fabric is, in my opinion, the "best" way to see if one's block will end up being approximately* the expected/hoped for size.
There are more variables than JUST the seam allowances that affect/afflict the finished size of a block.
*For me, a 12.5 inch unfiniished block should be at least 12.5 inches and no more than 12.625 inches after pressing. (12-1/2 to 12-5/8) It's easier for me to ease in 1/8 inch than to "stretch" 1/8 inch
With all the above said - there probably are some machines that feed the fabric better and will be easier to guide the fabric through.
There is a thread saying that after changing machines, it was ever so much easier to sew accurately.