Yes, the group knows what we need and tries to cover the bases. I'd consider it a must - though that does not mean it's a must from a player perspective. If the players are not going to do it, I try to fill in with either an NPC to fill the role or an item (for healing) which can mimmick the missing element.
It's all about adventure design, really.
The iconic four work best because they each fulfill a basic role during game play that accommodates general adventure design.
Can you get by without one of those roles being filled by something *like* the iconic 4? Absolutely. But from a DM perspective, your adventure will have to deal with the lack of a core group of skills in the party.
Because the iconic four are assumed by virtually every module designer, if you don't have them present in either PC or NPC form, you will certainly have to make changes.