The first first couple thousand feet above sea level do not bring on many changes. If you are doing basic recreational dives and staying well away from NDLs, it won't make a lot of difference, either. If, as Tom suggests, you are using a computer that adjusts for altitude automatically, then it will take care of most of the differences for you. The most important factor is that when we are concerned about decompression sickness, what we are worried about is the difference in pressure between the tissues in our bodies and the ambient pressure around us when we approach the surface and get out of the water. At altitude, that ambient pressure is less, so there is a greater difference. The greater the altitude, the greater that difference.
I will give you an idea of the degree to which this can be true by describing a reverse altitude incident. Years ago, I had an old Suunto Cobra, which had to be adjusted for altitude manually. (I think Suuntos still have to be adjusted manually.) I had stopped using it in favor of a different computer. I don't remember that sequence of events that led to this, but I ended up taking that Cobra (which is air integrated) and using it as a pressure gauge on a 2-tank, recreational dive in Florida. About half way through the second dive, it occurred to me that the last time I had used that Suunto was in the Denver area, and I had it set to that altitude. I immediately took a peek and saw that according to the Suunto, which thought I was diving at altitude, I was well into decompression, while the computer on my wrist (which knew I was at sea level) said I wasn't even close.
