I agree with AndiR, the more seams, the more exact you want to be, so to be safe, I like to use a scan 1/4" if it's a block that has many pieces to it. But for the most part, I use a 1/4 inch. Be sure to watch out for those pesky 1/4" feet that are a smidgen wider than a 1/4".
The best way I know of to measure is to use the lines on an index card or 1/4" graph paper. Put your needle down on the line, the line to the right of it will be 1/4" away, and hopefully the edge of your 1/4" foot will line up with that.