That's a cool jig. Another tip is make sure the neck is seated at the butt end too. Some clamps, depending on what you use, can exert a small amount of horizontal pressure. Make sure the butt of the neck is completely seated after you clamp.
What I have done is scribe a small line on the back of the neck where it meets the body. This is done during a dry fit. When I glue, I check the line for any separation. If there is a gap, the neck has slid.
Just is case I am not giving an accurate account, think of hanging a guitar by the neck and having the body slide down...this the type of gap I am mentioning.
Also be sure to have a damp rag handy to wipe off excess glue. If it sets, it is really hard to remove.