I'm guessing yes you probably should. If you internet searched for "zfs raidz boot drive freebsd install" you'd probably find enough to give you a definite answer. Or grab Michael Lucas ZFS and Storage Essentials books (lots of really good stuff in them). One of the devs that work on the FreeNas stuff at ixSystems can provide a definitive answer.
This is a little bit of an older link and talks about how to do an install, but is still relevant
My opinions below, based on me mucking around at home, not doing anything for big data storage/performance.
Everyone and their use cases are different, but for me with home use, I like using mirrors. Separate stuff for the OS and user home/data directories, a pair of Samsung EVO 850 SSDs don't cost much, so go put them in a mirror for OS/booting, then WD Blue drives in mirror or RAID to suit your storage needs. Heck the way ZFS works you could go and create RAID-Z out of mirrored pairs just because you can.
Oh, ZFS is great at using a whole disk, but when you start mirroring/RAIDing you need to remember that one makers "TB" is not the same as anothers (powers of 2, powers of 10), so even if you use the whole disk, you may want to actually use gpart and put partitions on them, so that you control where they start (-a 1m) and the size of the partitions match across devices. ZFS nicely uses partions instead of whole devices as you're building up things.