re: Settle an argument: Which is preferable, including a leading zero or not?

Leading zero. In the real world, it is proper to lead with a zero to avoid someone misreading the number by missing the decimal. At least tis is why I think you should lead with a 0.

I typically write -0.21 so the decimal isn't overlooked and confused for -21

I'm an engineer and throughout school to this day I can't say I am consistent with or without a leading zero... I probably keep it at around a 60/40 split in favor of no leading zero if I had to guess.

If typing I will always use the zero though. It's weird.

An extra zero on the right side indicates a greater degree of precision. For example, 21.0 is not the same as 21 because 21.0 indicates that you are confident in your accuracy to within one tenth. 21 shows accuracy to within one instead.

Of course, on the left side that doesn't apply although I'd prefer to have the leading zero just to be sure the reader doesn't ignore the decimal. Finally, we need pics of you and the roommate scissoring.

In the medical field you use the zero to avoid confusion because if misread it could mean life or death. Use the zero.