I see options for hours, minutes, individual seconds, and then another box. How would I be able to put in my time that uses tenths? (Direct example: I want to submit a 3m31s43ms)

The game's moderators can enable it through the moderation settings. If it doesn't appear, it just means the mod/s don't think it's necessary to be that precise.

If it's not an option for the time, you can't give it by default, but you can always put the timing down to whatever preciseness you want as a comment on the run. Note that if you do this to a run that has already been verified it will need to be re-verified.

I usually put the full time with milliseconds in the VoD title and thumbnail, so whoever actually cares can easily find it.

The majority of the time it's completely unnecessary if it's RTA timing rather than IGT.

Some games, such as Super Mario Bros. are optimised enough to require timing to the RTA frame, but this is not the case in most games, and many runners just submit whatever their splits say with no thought to whether it's actually accurate. Yeah, I'm sure that "xx:xx:xx.342" is totally accurate in a centisecond game that doesn't even time to thousandths.

The only real exception in this case is when two runners are tied to the second and are adamant that they want to know which run is faster. In all other cases, I don't see it as especially important if the RTA is not analogous to the IGT.