Technically,..  you should...
... but this is the first vacuum petcock bike I've ever owned, and I never used the shut-off on any of my bikes unless I was working on them...

So,... it's not a bad habit,... but if you forget, the world don't come to an end...
.. or if you are lazy or forgetful, like me,.. just don't bother...

If your float should stick while you are parked,... with the petcock in the on position, your bike will make a puddle and possibly leak gas into the oil...
... but,... if you have it shut-off... it will still leak when you turn it on to go ride... and you will be on it and riding while it leaks...
I don't see how that's any better...
Now you are away from home, broke down, and have gas in your oil... 'cause you didn't see any leak when you took off...
... and the same goes for the stock vacuum petcock...  it will stop the leaking while you are parked, and let it go once you are rollin''...
Bugga'!... ...

I say,.. just leave it on all the time... A stuck float is a bad thing, but it's a bad thing whether you are stopped or moving...
At least if it leaves a puddle,.. You'll know...

A more conservative rule of thumb would be:

If you think your float needle may be leaking, immediately check your oil level --
if your oil level has gone up change out your oil.

In any case, replace a suspect bad float needle ASAP and reset your float height -- having a bad float needle is trouble on the hoof and is best AVOIDED by fixing the little sucker so you know it is good.