We all have to deal with glue. We go through the whole process of dry fitting and clamping up pieces to make sure everything goes together properly. We make sure that all of the joints close tight and that the project clamps up square and true. Then it's time to glue everything and suddenly it seems like all the rules have changed.
The glue grabs quicker than anticipated. Or the joints that closed tight dry suddenly seem to not want to go together at all. Glue squeezes out and gets all over your fingers which then leave sticky fingerprints all over the place. As more clamping pressure is applied to get the stubborn joints to close, the piece begins to distort and rack, no longer nice and square like it was during the dry run.
The fact is that applying glue does change things. Glue contains a lot of moisture which causes dry wood to swell as soon as it comes in contact with the wood. So the joint that slipped together so nicely when dry may actually be too tight after the application of glue. This is exacerbated by time. The more pieces your assembly contains, the longer it takes to get everything in glue and put together. So while you are applying glue to the last piece, the first one has been quietly absorbing moisture from the glue you applied five minutes ago.
As much as we all love a nice tight-fitting joint, it may be in our best interest to make the joints slightly loose to allow for the inevitable swelling the glue will cause. This may not be too desirable if we are talking about an exposed dovetail joint but if the joint in question is an unseen mortise and tenon, it will not hurt to make a "loose slip fit" that will still be able to close with light clamping pressure after the glue is applied.
I have also found that it's often best to glue up projects in "stages." This is not easy for me because I am, generally speaking, a person you would not think of as "patient." I have to reign in my urge to get the thing done and move on to the next project. So if I can figure out a way to get a whole table base glued up in one step, I tend to want to do it. But if I can control the tendency to be in a hurry and stage out the assembly, I always end up with a better job and much less stress.