I think all fixed wing pilots should have at least one lesson in a helicopter just to get an appreciation for just how different to fixed wing flying rotary flight actually is.
Heavens above, I hope you're not suggesting the corollary - that rotary pilots should have a lesson in a plank?
On a serious note, one aspect of which a fixed-wing pilot should be aware, is the responses required in an rotary emergency are the converse to those in a fixed-wing emergency. There has sadly been a few rotary accidents/fatalities involving a high-houred fixed-wing pilot but who only had recently passed a helicopter PPL. To that end, I would certainly advise having an instructor who flies both.
Cheers
Whirls