All you need is 89 Octane. That's what it was designed to run properly on. You can run 91 or 93 if you like, but it's not going to perform any better on it.
Some areas or states have no ethanol in the premium gasoline, so that may be the reason why your dealer said to run it. Ethanol creates lots of troubles in vehicles that sit long times between uses.
From a 2013 Owners Manual page 4-18: (the engine hasn't changed since '07 regarding fuel)
"Your Yamaha engine has been designed to use regular unleaded gasoline with a pump octane number [(R+M)/2] of 86 or higher, or research octane number of 91 or higher. If knocking (or pinging) occurs, use a gasoline of a different brand. Use of unleaded fuel will extend spark plug life and reduce maintenance costs. Gasohol
There are two types of gasohol: gasohol containing ethanol and that containing methanol. Gasohol containing ethanol can be used if the ethanol content does not exceed 10% (E10). Gasohol containing methanol is not recommended by Yamaha because it can cause damage to the fuel system or vehicle performance problems."
Ask your dealer to show you an Owners Manual and then show them page 4-18 or just look up fuel in the index. Feel free to call them an idiot, Lord knows there are plenty of them here in the Commonwealth, and then find a new dealer and feel more than welcome to ask additional questions here.
87 pump fuel will work fine in your Grizzly. If you are getting the E10 and let it sit in between rides then I'd recommend adding a couple of ounces of stabilizer to your fuel.