If you've got the time, I would just study new cards instead of worrying about studying old cards before they're due.  The SRS system is designed to optimize your ability to memorize and retain huge amounts of information in the shortest amount of time possible, by only bringing cards up for review when you're close to forgetting them.  Extra reviewing can be counterproductive to this.

I would not review more than this site sugests, as it might ruin the process of transfering the memories to long term memory. A part of remembering is forgetting.

I know mnemosyne is prepared to do this, but he only allows it after you have done all your cards.