I was told by someone that the original bearings used in MVVS engines are low quality bearings and should be replaced.

Was this ever true with MVVS engines?

I own an MVVS 77 that was purchased about 6 years ago. It is NIB. Should I replace the bearings?

And how does the MVVS 77 engine compare to the OS 91 FX?

One of the reasons I like MVVS engines is that they will run on low nitro.

My friend runs 0% nitro in all his engines with good results. 22% oil (2/3 Klotz - 1/3 Castor).
(with this fuel it is very hard to damage an engine if you have a very lean run).
I run what ever fuel I can get my hands on... but I will add oil if needed. (cool power fuel I will add about 6 oz of castor)
But I like low nitro fuels the best. Or no nitro. I used to make fuel from scratch.. but I don't have a place to store methanol anymore(50 gallon drum).