Rohloff does not recommend a specific gear ratio, they specify a ratio as a minimum so that you do not put too much stress on the internal components.
http://www.rohloff.de/en/news/news_r..._ak/index.html

Look at it this way, the Rohloff gives you very even spacing of the gears from 1st up to 14th gear. Thus, the best way to decide what ratio of chainring to rear cog you need is to figure out if you want a gear higher than your existing 14th gear - - or lower than your existing 1st gear. If gear 14 is too low, you need a bigger chainring or smaller rear cog. Alternatively if your gear 1 is too high, you need a smaller chainring or bigger rear cog. If you never need a higher or lower gear than you have, then life is good and no changes are warranted.

But, stay within their specified ratio. I do not know if it would void the warranty or not, but why risk it?

In flat areas, I spend a lot of time in gears 10 and 11. Probably less than 5 percent of the distance I travel is in gears 1 thru 7.

I am running a 44t chainring with 16t cog. When I tour in hilly areas with a full load of camping gear, I bought a 36t chainring to use instead. When I have a heavy load on the bike, I will need the lower gear and will be going down the hills slower (for safety) so I will not need the higher gears that I otherwise would want to have.