Not very, in my opinion, but people have developed problems with their 770s when doing it. Personally I feel it is not so much that they have been flashing their device but that it was prone to breaking anyway and the repeated reboots forced the issue.

I have so far flashed the 2006 OS onto mine, then gone back to the 2005 OS because I needed these third-party ARM packages to be happy with the 770. After that, I've also flashed the device into R&D mode and then (after altering the gainroot script to ignore whether or not the device is in R&D mode) flashed it back out of R&D mode and have had no problems.

You have no choice if you want these things to work, though. You have to become root and install the package(s) with "dpkg -X packagename.deb /" if you want them to work right off the command line, and you have to be root to run them if they need full access to the 770.

Easiest way to do this is to download a Linux Live CD that you can boot and run Linux from without tampering with your hard drive, and then run the flasher-command from the command line there. If the Live CD is very current (kernel 2.6.something or higher), you'll also have to run a perl "fix" to make the USB connection work right.