The ratio totally depends on the race course. I raced in over 30 local and regional events last year all on my SS and each race I did an assessment as to what gear ratio I would run.
My background = Cat 2/Sport racer for first half of 2009, then bumped up to Cat 1/Expert for the 2nd half and that's where I will race this year. My most common gears I supposse were 34x17 and 34x16 for the flat, sprinty stuff and most things under 2 hours. (i.e. at Sea Otter last year, I got a 4th in Cat 2 SS on a 34x17). But for 8-hour events and hillier stuff I'll be on 32x19, 32x20 etc. I think 32x20 is the lowest gear I used for a race last year.
I raced a cyclocross race (lots of long flat wide open spots) this week on 36x16 and did well. My chain actually popped off after the first lap, and I had to stop, flip the bike upside down, fiddle with it, losing over a minute, and that gear still allowed to catch back up and take 1st in Open SS.
Also, it's obvious to gear low for hillier races. But one mistake I found myself making this year was I would not take into account for "twistiness or tightness" as well. Even if a course is flat with little hills, if it's a very twisty course where you'll be slowing down a lot for corners and them stomping on the gas a lot, gear lower than you might think so that you can accelerate fast. I've made the mistake of saying "oh this course is flat, very little climbing, so I can gear huge today". But then I'd find that I'd lose time by not being able to use that available top speed from the huge gear by being bogged down in the tight stuff the entire race.
So it's a tough question to answer.....very specific to the individual rider, the course layout, and the conditions of the day.
