I think snap radius starts at 20 by default. Are you sure you have snaps activated, just selecting the tool from the drop down menu doesn't always activate them. the snaps marker is is just a visual aid, and i think that is set to 30 by default.
One way that helps me line up very simple unwraps like walls, is when editing your UV's in the UV Layout dialog in the upper right corner you can select a different texture to be displayed on the background of the layout editor (instead of the checkers) from that dropdown choose "Pick Texture"...then Bitmap from the…