When you installed your bottom bracket, what specifically did you grease? You should have greased the inside of the bottom bracket tube on your frame, including the threads where the bottom bracket cups screw in. Grease the inside of the cups, where the bottom bracket shell sits. A light layer of grease on the bottom bracket shell itself (to prevent corrosion). Install the bottom bracket to the recommended torque specs. Thoroughly clean the square tapers on the spindle and square holes in the crank arms (square tapered crank attatchment should not be greased, which is different from splined cranks which are greased). Install the crankarms. Grease the threads of the crank bolts and torque them down to manufacturer specs.
If this is how you installed the bottom bracket, the creak is likely elsewhere. There is a small chance that the bearings on your bottom bracket are making that noise, but if it is an XT that has never been installed before, chances are very slim that is the issue.
As a minor point, copper grease was primarily designed to withstand high-heat. For mountain bikes, copper grease is useful where applicable on disc brakes. I am unsure if copper grease has the potential to harm rubber seals, so a more generic bike grease might be useful for generic bike applications(many mechanics choose a lithium based grease).
Cheers,
Juba
