basic NULL ??

Among many other duties I administer a database. we've used informix since about 1986 ver. 2 i think. in 1992 we upgraded to 4.00 . in the beginning it was recommended we stay far away from NULLS. upon upgrading to 4.0 the not nulls in the table descriptions worked fine so I left them in. Now moving to ver. 7.0 with isql & se and table descriptions with not nulls doesn't work. wondering what the latest opinion on nulls is. should i continue to avoid them? was that initial advise bad? if nulls are now fine and I allow them, are there cautions I should take? I still consider myself a newbie even though I've been at this awhile. Thanks, Paul