I have been keeping my line on my baitcasters much less than the full that is normally (recommended. (1/8 inch from the top of spool). With this lowered amount on there, I can almost spool it all out on a long cast. I find I still get long casts with better accuracy, and less chance of a professional overrun, (birdsnest). Will this hurt my reel, and what are the negatives to running less line than normal? Thanks.

Less line means your reel isn't going to retrieve as fast as it would with a full spool, other than that, you'll be fine.

it won't hurt anything. 

 

Tournament casters spool their reels with the distance they expect to cast, plus an extra wish - they cast all the way to the end.  They don't care about backlash at the cast end, and the less extra line, the less they have to clean up. 

By keeping the mass down even a little bit, the spool is faster you get equivalent braking at a lower setting. 

Besides what mentioned above, you might see a slight decrease of long cast distance. The less line on the spool, the smaller the "actual" spool diameter is, which means less line payouts per turn when you cast. So, depending on how you cast, there may be less backlashes, but at the cost of max distance.