Go ahead and do it. Even if you "brick" your device by deleting something important (or rm -rf /), it can be reflashed without any troubles. So not actually a brick.

There were some warnings associated with setting the R&D flag, but this was because Nokia were having screen problems (WSOD) and they would tend to fail when you reboot (which you have to do to enable R&D mode, and probably do more often if you're hacking and have enabled it), the WSOD isn't caused by the R&D mode.