I'll soon be getting a new ball drilled, and I have a question about spans, cause I have a feeling my current span is too short. So I've always heard that the last joint should be aligned with the center of the hole, and if I put thumb in the ball and relax my hand, this happens. However, I find that I can stretch my fingers so that the last joint is at the top of the hole, without having my thumb change. Is this how I should be bowling? I feel like it may help me stop squeezing with my thumb.

Even with a stretched span your palm won't be flat on the ball.  About gap about 1/16" will still be there.  Typically, in your case, you should add tape to snug up your thumb hole and/or try more forward pitch to reduce squeezing.