There are a lot of words that could be used to answer your question.

I think that the best way to put it is by stating requirements and properties.

The glue that you'll be using is not intended to fill gaps. (TB I, II or III)
The glue requires that the surfaces be in contact with each other.
The tenon should be a snug fit into the mortise, to the point that a few gentle taps from a rubber mallet is needed to disassemble a dry fit.

BTW - A couple of tips.
Don't put glue on the tenon but rather in the mortise.
Make the mortise deeper than the tenon. This leaves room for the glue and allows the joint to close. (Glue is NOT compressible.)

 

When I do M&T's they are a slip fit with slight resistance. I make a slight chamfer (1/8") at the end of the tenon so it will be less likely to scrape the walls of the mortice of glue when inserting.

I also put a very thin coating of glue on the tenon and brush it damp to almost dry, and then apply a thin coating of glue in the mortise. A well fitted M&T with glue can be like a piston in a cylinder. It may not seat all the way in, or may get pushed out by air pressure. Dowels can do the same thing unless they are flutted or scored. I usually knife cut a few grooves in the tenon to let out the air.

 

How tight is tight is a tough to put into words. Start by saying that before you glue, if the joint falls apart under it's own weight, it is way too loose. If you have to beat it in place, it is way to tight. What you want is somewhere in the middle of the two. You should be able to dry fit the pieces by hand with a moderate amount of pressure. Again, its hard to describe 'snug' in words. Even just a slight nudging with clamps is OK as long as you dont have to crank down on them.
A glue joint is effective with the glue at about 1/1000 inch thick. That aint much. Also, when you glue up, what seemed tight when dry might be easier with glue as the glue acts like a lubricant when wet.
If the joint is too loose, normal glue will not act a 'filler' and the jpint will fail. If too tight, you will force almost all of the glue out and end up with a 'starved glue joint' which will fail over a period of time. Also if too tight, you risk splitting the mortised piece.
When it's right, you will somehow instinctively know it.

 

I agree with all above and also use Cabinetman's chamfer. Also I use a thin flush-cut saw to saw shallow slots across the end of the tenon for glue to flow through.

The mortise should be deeper than the tenon, but not by much. Just don't put too much glue in the mortise. Use a thin coat.

 

I want to get my joints together by hand without pounding them with a hammer . . . but just barely.