So WT likes tight fitting joints because hide glue has poor gap filling 
qualities.
Let's define tight.
If tight means pounding the tenon into the mortise with a mallet then 
there would be very little glue left on the joint surfaces to be of any 
consequence.
Let's define gap.
If gap means the tenon just drops into the mortise with no other 
assistance then it is a sloppy joint and even though the hide glue has 
poor gap filling qualities it will still hold in the few places where 
there is some contact between the surfaces. So it still helps a badly 
cut and fitted joint, no matter what.
On the other hand a well fitted joint comes together with firm hand 
pressure and some body weight accompanied with some gentle wriggling of 
the pieces. This allows a film of glue to remain between the surfaces 
to provide the extra support for the joint. The glue should only be at 
the shoulder of the joint to allow the rail to move with humidity 
changes.
Now lets define the surfaces of the joint.
The rougher the surface the more surface area so the better bond. BUT 
there is a limit. If it is too rough then there will be gaps between 
the surfaces and poor adhesion is the result. Paring the sides or 
cheeks of a mortise or tenon means a cut across the grain which even 
with a really sharp chisel will result in a mini plowed field effect, 
an ideal gluing surface. Normally the chopping of a mortise with a 
chisel results in a reasonable gluing surface with no other paring 
necessary. It is sometimes essential to pare the cheeks to eliminate 
winding in the joints, but this should be done in a way so that the 
joint doesn't become sloppy. Normally it can be accomplished with 
minimal paring of opposite ends and sides where a few thou' will remove 
a 1/8" wind quite effectively.
When it comes to ensuring the efficacy of a joint, as in all other 
things, moderation is the key.
Peter
Visit my site at:
http://www.oldtoolsshop.com/Galoots/pHyde/
Use it up, wear it out, make it do or do without

------------------------------------------------------------------------