Spend the first few nights in a hotel, and after your visa is registered and passport returned to you, move to your flat. But if you live in a flat without being registered there (a whole different complicated process requiring a special spravka from the housing office in charge of your building)it could be a problem if the cops ask to see your documents. So don't hang out at tourist or expat bars and clubs, don't be drunk in public, and don't go in the metro late at night (all easy pickings for cops looking for extra income).