I have always used Sharpies (mainly because they are cheaper), and I've never had a problem with bleeding. I heat set all the writing, with a dry iron on the hotest setting for that fabric.
One suggestions In order to keep the fabric from shifting around while you are writing on it, you can do one of two things. (1) place a piece of extra-fine or fine grit sandpaper underneath the fabric while you are writing or (2) iron freezer paper to the wrong side of the fabric. If I'm doing all the writing, I use the sandpaper. If I'm doing memory blocks for other people to write on, I use the freezer paper, and cut it so that it's at least 1/2 inch smaller than the block. That way, the signer won't write all the way to the edge, whcih would be lost in the seam allowance when the blocks are sewn together. Of course, if the area they are writing on has a border around it, you don't need to worry about this.