For a bathroom tileing project, I have finished laying down backerboard and taped and mortared the joints as the books tell me to do. After letting it dry overnight, I'm left with raised mortar lines (aprox. 1/8 inch) across the otherwise smooth and flat prep surface. I can't help being concerned that this will cause problems when attempting to lay the tiles on a fresh bed of mortar. I used just enough mortar to cover the tape strips, but is it possible I still used too much? Is this a valid concern, or should I trust that the fresh mortar will fill the uneven gap left by the previously dried mortar? My books don't address this concern. Thanks.

The fresh mortar should smooth evering thing out. The one thing I do is when applying the fresh mortar always trowel with the joint. In other words don't trowel cross ways over the tape joint but rather along side it as you would with a drywall tape joint. If an 1/8" is all you have that is fine and you should not worry. Make sure to run an old trowel or puddy knife along the tape joints before you start. This will asure you have no little burrs sticking to the floor. Those little burrs will fight you when it comes to having level tile.
Mike

If you have a actual 1/8" raise then you put too much mortar down but you should be OK. You are using a modified thinset and not actual mortar, right? How large are your tiles and what size grout line are you using. Mant 12x12 tiles will have you lay your thinset with a 1/4" trowel and that will help the 1/8" disappear. I will tell you how I lay Hardibacker even though it is contrary to the Hardie Company instructions. I do not use a tape application in advice of laying the tile. I tape and apply the 1/4" thinset pretty much all at the same time. I figure the 1/4" thinset is more than enough to set the tape, quicker, cheaper and no 1/8" bump.