The worlds you typically 'roll up' are the major world per system of human interest, activity and mostly population. As such they skew towards some level of breathable oxygen atmosphere and a few vacuum, exotic/dangerous atmospheres or asteroid belts.
It's probably best that you at least figure out the temperature of the place, derived in large measure by the distance from it's star plus the atmosphere. That will also drive the biosphere, or at least one functional for Travllers to interact with.
For all intents and purposes then, these are civilization or outposts of civilization, most of which will be at some level of inhabitable.
However, consider fleshing out minor worlds, the OTHER worlds in a system. Those will be methane/ammonia or worse atmospheres, iceballs, moons around gas giants etc. They won't be your trade or outfitting entrepots, but they may hold secrets and undiscovered treasures and hazards. May be no known humans, but that doesn't mean no one is at home....
I like RTT Worldgen for that purpose. Historically the Scouts supplements have had system builders. I recent got hold of a World Tamer's Handbook, am kind of ignoring the RC/low TL colonist bits and warming up to the planetary definition portion. There are other supplements I haven't seen that get into this arena, specifically using the planetary profile to help flesh it out.
Now then, as others have already noted it's a function of what content you want to provide your table. You can adventure quite a bit at one planet, however it IS Traveller and ships and going to new planets is part of the scene.
I would recommend pregenning a few region types (mountain/desert/plain/river/ocean/exotica) and just plunking them down on planets as needed, adjusting for temp/weather/atmosphere etc. You can do a planet's worth of definition but use the regions across a subsector.
Best of both worlds, as referee you should be striving for giving them that 'not in Kansas anymore' feeling, and the little details like 'perfectly breathable atmosphere but the star lillies release pollen that causes cancer, wear that respirator' really helps the suspension/immersion.
Just as importantly, a place is also people, so you want to give some thought as to what sort of culture is there, what their values are, and unique setting bits like what do they eat/drink, ignore/care about deeply, how badly they want to work with or kill each other and why, and most of all what can you sell to or buy from them.
