We have two Kubota dealers both about 30 minutes away from me. One installs a block heater on every single Kubota they sell, and charge the customer for it. The other dealer doesn't recommend them. At this point I've had four Kubotas, and they have all been stored the same way, in the cold, without a tender or block heater or anything else...and they've all started well. My 2002 L4310 had an automatic glow plug timer and started very well all winter, each winter, right up to when the battery died in *2018*. Very impressive I think.I think it is dependent on which tractor you have, and how old it is. I have run across Kubota blurbs about redesigned glow plug chambers etc for easier starting. I have no clue as to which models have what. My two, 2013 and 2016 editions are easy to start, dealers told me don't bother with block heaters, and I haven't. If I am not running them long enough to recharge the battery, I leave them on a battery tender and that has been fine. The battery tender use did increase cranking speed on L tractor, but that is an older battery.
I'm very fortunate that both my current tractors are very new, and I expect they'll start well for years to come with no special attention.