For some things, 0 and A and absolute limits: I cannot see a negative hyrdographics value, nor anything over 100% for that. Most of the other fields are really lists in some sort of order, so really the values are more of a key to the value and not the value itself, unlike the hydrographic code. Well, population and size: cannot have a negative population nor smaller than 0 which is a belt and could be as small as a single rock or something like Glisten and spread out. Atmosphere 0 is a vacuum, so I cannot see anything less than nothing for that field either.
Okay, so the 1st 3 4 physical stats I cannot see going below 0 as they are directly corollated to a physical aspect, and 0 of anything (atmosphere, size, hyrdo) is the lack of those things. And population cannot be negative. So those "keys" represent a numerical value or percentage.
The TL I cannot see going below 0 either: while we like to link the TL to eras, it is never a 100% clean match. and 0 means no technology so not sure how we could have a negative technology.
Now, with that, it would be interesting if those values, though limited to 0 in the UWP, could still carry over the actual value (-2 for example) as a DM where applicable. Been a bit since I've done world generation, but in a lot of other systems I use. there are effects that carry over from other rolls even if you have to round up the final value to fit ti final product.
And sorry, I seemed to wandered all over. Early still...
In answer: where things cannot be below 0, round. But if they can be below 0 and have a latter affect, keep the value for the DM but for the LBB3 world it will be listed as 0. Effectively an invisible DM once you are past generation. That's how I'd handle it (almost like the T5 genetics wher you keep the 1st die, there is a "hidden" value for those values not allowed to go below 0 but can based on DMs and such)