Fragmentation Level for Heaps
Heaps have a few special challenges that you can't experience with clustered indexes:


Forwarded Records
Captive Pages


I'd suggest running sp_BlitzIndex against your database to find out if either of these things is happening with your Heaps. If not, then leave them alone. If they are, you may need to consider rebuilding them. 

At this time, you can't reorganize a Heap table, and rebuilding a Heap table will also rebuild any nonclustered indexes on it. It may be cheaper to drop them, rebuild the Heap table, and then recreate the nonclustered indexes afterwards.

You can read more about this stuff here:


sp_BlitzIndex Self Loathing Indexes
How To Fix Forwarded Records
Mysterious Forwarded Records
Forwarded Fetches and Bookmark Lookups