Nope, I don't keep spares on my Home NASs (2 of them). If a drive fails, I can go a buy one same day. I don't see th point of having money sitting on a shelf.
For a business environment, It all depends on what you are storing, how important it is and if it's backed up.. Generally I like to keep multiple hot spares in an array, but I'm not talking about small arrays like a Synology NAS, I'm talking large Enterprise SANs.
As for raid 10... I've never been a fan, personally I run everything RAID-6, even at home.