Use the magnification tool when editing.. and make sure that you can't see a line after you are done your tile!

I think what often happens for new people is when they are blending edges (read: using the blur tool) they are ALSO blending the 15 pixel border along the edge of their tile... so when WORKING on it, it is blended.. but when uploaded and the edge gets replaced with the original edge... it looks like crap.

Go for it.