87 can be run as long as there's no pinging. Pinging (pre detonation) will damage the engines. Every engine varies, so one person not having pinging issues does not mean you won't have pinging issues. There's also other factors that influence the likelihood of pinging to occur- higher ambient temps, cooling system efficiency, engine load (towing or heavy throttle), etc.

Facts aside here's my opinion:
If your vehicle is running well on 89, and you're not putting a lot if miles on it (ie not much cost savings), stick with 89.