You can post at any frequency you like. One thing to consider is that if you tend to post with a high frequency, there is a greater chance that you will posting something that has already come up. A courtesy search to see if the issue has already been reported would be much appreciated by the devs, but if nothing comes up then post away (no issue is too minor, if they are too busy to fix it it will just be shelved for the moment, but the important thing is that it was indexed).
I typically use normal priority. The exception would be a crash that happen on a pretty common event (like game start, or adding ships to a cue). Otherwise they already try to get through everything, so I don't try to push "my" report as more important by giving myself some grand priority.
