It won't hurt anything to use a splitter. As you said, the two inputs are typically summed back together anyway and if you are using a single output, then you are feeding it a mono signal as well. Where it can come in handy is if the receiver has a low preamp output level and the sub's amp wants slightly more voltage. In this instance, the sub will go to "sleep" during quiet passages because it doesn't think it is getting signal because it is too low. I had an HSU sub that was like this and they said to use a splitter and it worked. If your sub doesn't have this issue, then a splitter probably isn't necessary.
