One thing I have noticed over the past several months in seeing many winning bids for my clients is that it appears that in at least many cases that you stand a much higher chance of getting your bid accepted even if it is a low bid if you are bidding on the next category up as opposed trying to bid on a category 2 or more away from your current category. This makes sense as they can then offer your cabin then the one below it and so on.
For example, on a lot of recent sailings a lot of my clients won a bid from an OS to a CLS with a minimum bid. Pretty sure there were lots of other bids on the CLS from lower categories with a higher amount but the more cabins that they can turn over the higher the overall extra revenue they will achieve.