Is fire hardening the tip worth it?
No - except for certain highly resinous woods like Eucalyptus. Fire treating the "point" will change the chemical composition of the resin(s) to form complex, cross linking polymers which are both hard AND tough. This unique combination of characteristics significantly improve both the wear resistance as well as the ability to "take a point".
Is it really necessary to polish the tip of my Pointy Stick
No.
or can I stop at 6000 grit?
Actually, you can stop at 320 and then, using a hardened steel or agate burnisher, burnish the "point". This compresses the wood, making it both shinier AND harder - sort of like case hardening. This will require a bit more care to avoid "rounding over" your "point".
But hey - if you want to go to 6000 grit - by all means do so.
PS - there is no evidence that cryo-treatment does anything to improve Pointy Stick "point hardness".