The only heirloom I've bought was leather.
But then again was only planning on shammies and druids, possibly a rogue team, at that point.

I'd probably recommend cloth shoulders and cloth chest heirlooms for the teams.
One of each per account, and then anyone can use them.

Cloth is worthwhile, even on a plate toon, for the experience boost.
With 5x toons, it won't make much difference.
And that way, anyone can wear the things, even your new team of <insert class> that you make spur of the moment, sometime down the road.