Instead of your speaker cones going in and out, they'll go out and in //content.invisioncic.com/y282845/emoticons/laugh.gif.48439b2acf2cfca21620f01e7f77d1e4.gif
You won't notice a difference with just one speaker, but if you have dual subs for example, they should be on the same setting as each other or you'll lose bass. You should also have left and right going the same way as each other or you'll lose imaging.
If you are crossing over a sub to mids, try it both ways as crossovers shift the phase and can cause cancellations. In this case, just use which ever sounds best.