doogle, I'm not completely sure about your visa situation, but when family came to visit last summer they had 7 working days to register. I don't think it will be the end of the world if you don't, but I suggest that you try to do so if possible--maybe a night or two in a hotel?
Agree with Walter on the dates on the migration card; while I'm not sure if I've ever left later than the stated date (I usually insert a date far in the future), I've never heard of anyone having a problem because of this.
If you want a second (third?) opinion, there's a Russian expat forum called expat.ru which has a subforum on visa issues, if there is not an existing thread on the topic (and there probably is) then you should be able to get an answer quickly if you ask.