I've rarely used glue on my projects and I find that I honestly don't need to. The joints are so strong that honestly, glue is optional. It's always a good idea to use it, but like I said, rarely do I use it. Never had a break down yet on any of my many projects! ;-) Also, it may help when you do choose to use the glue to either take your finger and slide it down the glue trail, or use a glue "brush", this helps with the sliding around - doesn't get rid of it completely, but it does help! :-)
I agree with tracysmith. I do use glue with my PH connections whenever I can just because it is a more secure joint. If you are joining a 1x2 to another and only have one PH in the end of the 1x2 I would always use glue just to keep it from rotating out of line during the final build. However it isn't necessary. If you plan to stain minimize the use of glue because you will miss a spot and glue does not take stain. Also when you do use glue make sure the entire surface is covered. I do it with my finger while others use a brush.
The glue is totally optional - in my case, sometimes I use it and sometimes I don't. As for the "sliding around" when using glue, as soon as the pieces make contact I wiggle them around to create suction then they won't slide. Does that make sense? Hope this helps!