HAIKU for PEOPLE is a nice site to check out what Haiku and related terms mean. I specially try to incorporate the seasonal element in the few haiku's/hokku's I try to write.
Many people feel that one should not have to follow any rules as this is art. I say only two things,
one, to bend rules effectively you need to have had worked rigourously under those rules for some time, and two, if you do not follow this rule why call it a Hokku/Haiku. After all names are classification based on some rules.