There are only two situations that I can think of in which you would want to consider not upgrading wordpress:
If your custom theme is a modified version of one of the default themes (twentyeleven/twentyten).
or if your theme is using out of date hooks/actions (which if you are upgrading from anything above 3.2 I’d be willing to be you’ll be fine) you can go here http://adambrown.info/p/wp_hooks to double check.
If you made the theme yourself or customized someone else’s theme (other than a default) and you are not upgrading from a very old version of WP I’d be willing to bet that you will be fine. I’m sure other’s will be able to voice their opinions if I missed anything here.
But remember always backup! just in case.