Its good to go up a few teeth on the back equal to one down on the front if you are changing out the whole set.
+1.
Changing the front down a tooth is the same as changing the back up a few. Most people do the 14 front change because you can do it easily without removing many parts. To change the rear sprocket is more involved. But if you are putting on a new chain entirely, it's only a little more work to swap out the rear sprocket.
Why do a larger rear rather than smaller front? General thought is the smaller front bends the change more and causes it to wear faster. I don't know how much data is out there to support this theory, but I buy into it. My bike had a 14 front from the PO and when I bought new wheels I decided to go back to a 15 front and increase the rear to 42. It is roughly the same ratio as the 14/39 I was replacing.
There is a good webpage about drivetrain numbers that is worth reviewing. I think it is Brad Black's site, but I can't find the page I'm thinking of. I will post a link if/when I find it.
[Edit - here it is:
http://www.bikeboy.org/ducgearing.html As you can see in the chart at the bottom of the page, my 14/39 that the PO had installed is nearly identical to the 15/42 I put on when I changed wheels. Of course, if you are looking for tighter or looser gearing, the chart provides a nice sense of what the effect is and how it compares to your current setup.]
