Most of the hacks will survive an upgrade or a factory reset (such as the jailbreak itself, or localization).
A few would be partially overwritten/erased on a factory reset, rendering the hack itself useless (such as usbnetwork or font hack), but leaving the device functional. This can be easily fixed by simply reinstalling the hack.
Finally, very few (the orientation hack in a few of its variants, if I'm not mistaken) might prevent a successful update altogether. In this case, there are prominent warning messages in the threads.
Conclusion: just try it.
