Initially to get your ratio up, I would recommend exchanging your SBP in increments of 600 SBP (best value) for upload credit. Once you build your ratio up enough, I would be inclined to use them for other things such as helping out members who upload the torrents that you download that you really like. If all else fails, you can always let your SBP accumulate over time so that if you're ever in a pinch because you went on a downloading binge, you can help offset that.
Alternatively as you mentioned, you can also make torrents FL. This can be particularly helpful in a number of ways once you have built your ratio up. For one thing, you can use this advantage to be able to snatch a torrent that is near death because it's not being seeded, download it, then keep seeding it and make it FL to give other users incentive to download the torrent. This both helps to keep older torrents going and it also gives you an added advantage of being able to improve your ratio indirectly with SBP. If you play it smart, it really is a win-win situation.