I’ve definitely had a change of heart on this topic; over time we have seen RAM fail, both ECC and non-ECC. So I’d advise ECC whenever possible on servers.
It’s basically a game of statistics. Though my mini-pcs I have colocated for 5+ years have never had issues, the more servers you have, and the more time goes on, the more likely it is to happen. 
Cheap Insurance is the right call – if you’re colocating just a server box or two, I wouldn’t agonize over it, but if you plan to colocate dozens, all ECC all the way. 