Are there any rules for de-gas breads?

Usually, doughs for breads where big holes are appreciated, like ciabattas, baguettes, artisan breads so to speak, you want to try to preserve the big holes(bubbles), by handling the dough as gently as possible(practical). No real degassing.

Doughs for breads with a smoother, even crumb, like sandwich loaves, burger buns, etc., big holes are usually not wanted. You can be thorough in pressing (knocking, whatever you want to call it), the dough to make all the bubbles small and equal.

From my limited breadmaking experience.

One aspect of bread making that I appreciate is that there are very few "rules".  Where rules do exist, they beg to be broken in the interest of discovery.  The general rule of thumb for degassing is well stated in mrfrost's comments.  You will degass your dough to some degree every time you handle it but it is important, for some specialty breads, to avoid that as much as possible (e.g Ciabatta, Focaccia, etc.)

There may not be any rules, but there are consequences. Like previously stated, how wide open your crumb is can be dependent on how much or in what fashion you de-gas your dough. If you want an open crumb, you don't knead the bubbles out of your dough, whereas if you don't want an open crumb you do.

I think the most important consequence of deflating the dough is that you are redistributing all the flavor that is produced during the fermentation when all that gas fills up the dough. In my opinion you should always "de-gas" your dough, but you needn't always crush your dough. 

There are no rules, but by de-gassing you are limiting the openness of your crumb and distributing flavor, decide to de-gas accordingly.