I asked about this at several pilots, seems you still have to get 50 gal. but if you can show your reciepts for 50 gallons in the same week, they will give you a shower.
Fact is that unless the fuel desk operator is in a bad mood, or a shower nazi, you can usually ask for a shower and explain that you only have a 30 gallon fuel tank, they will give it to you (usually).
Bob