If I seat the bullet to the cannelure, the COL is much less than what I have been loading for my rifle. What effect will the cannelure have if I do not seat the bullet deep enough for the mouth to contact the cannelure?

None at all....The cannalure is for a crimp only and has no other service..just ignore it.

I was wondering if anyone noticed a pressure differance between crimped or uncrimped loadings everything else being equal ?

You can try crimping into the cannelure to see if your accuracy improves. Many have used the Lee factory crimp die and have had better accuracy with that than no crimp at all.

Thanks for the responce. Clint

Crimping into the cannalure eliminates bullet creeping in
compressed loads and tubular magazines where the bullet is
under pressure and could possibly change the seating depth.
If neck pressure is correct this is not a concern with most
box feed type magazines. With 154gr Hornady Interlocks, for example, I set the bullet seating depth and case length so
that the crimp into the cannalure is automatic when seating
the bullet. This requires each case to be trimmed to the same length and is good loading technique anyway. If you are not loading a bullet that has a cannalure, be sure to
set your seating die so that the case mouth doesn't touch
the crimp area of the die.
I have loaded cannalured bullets both ways (crimped into the cannalures and not crimped), and I can't see any accuracy difference either way, however for hunting purposes
I prefer the crimped round for stability. This works for my
rifle's chamber, but you may get better accuracy by seating
your bullets out past the cannalure as rifle chambers are often cut different. If you want to seat below the cannalure, it should make no noticeable difference. BLR7.

Years ago a very good reloader with lots of experience, Stan Watson, did a ton of testing on 30-06 with many powders, bullets, loading depths, temp changes, crimping etc etc. He wrote a book of his results and most of his data was backed up with a pressure gauge. I bought his book which is a "must have" for any avid reloader. HIs findings regarding seating depth and crimping were changes of about 2% which is basically insignificant. On a 30-06 load that was doing about 45,000 crimping took the pressure up to just over 46,000 psi. Seating depth was about the same unless the bullet was really deeply jammed into the rifling and that caused about a 5-6% raise in pressure. He did note that crimping seemed to make for more consistant velocity but not a "gain" in accuracy at the target.