On some steel frames you might find they flex a fair bit when used as a singlespeed. No problem so long as you are using SS cogs and chainring, but if you are using derailleur stuff, then the flex may cause the chain to come off.
Also, when you’re climbing you may find the frame flexes enough for the wheel to rub against the rear brake pads. Levers with the little slackening thingie are useful here – so long as you remember to put it back before going downhill. 😳 If that doesn’t work, you’ve discovered why fixed wheel was more popular than singlespeed 60 years ago. 🙂
(The flex isn’t a fault – it’s a feature and is why those bikes feel so nice on an all day ride)
65″ was generally regarded as the useful gear for an all purpose bike.