well this is something thats completely avoidable however you have to notice it before it becomes a problem, much like the problem that people have with the recompression of the JPEG2000 format useing the BMP format may produce some problems as well.
in useing the normal Maya sculpt map script and the Uber Sculpt map script exporting a very low resolution BMP (default 64x64) produces some edge bleeding useing a system default color (black or white) which washes out the values to a point where it can and will distort a model when committed to the inventory.
I have also noticed that when BMP is used the surface is a bit lumpy because of poor gradients (compensated by dobbeling the size of the image usesualy corrects)
I have found that the TGA format dosent have any of the afore mentioned problems and dosent require larger sizes to acomplish the same sculpt map. where the result may be a little lumpy its usualy due to the recompression by the server, or so i have noticed.