I wouldn't worry about it too much honestly, on the other hand you could consider a "skills degradation through "rustiness" system to offset some of it.
just thinking for a moment but what do your chars actually do during travel "down time"
for most techs, they are likely doing maintenance, repairs, and the like unless the equipment was "mothballed" for long term storage, so no biggie there.
but what about mechwarriors fighter pilots and armor crew, I mean they CAN use their mechs, or dedicated "sim pods" if they have access to them. but for some reason I suspect most transport commanders would get very cranky about firearm training and range use for some reason....
so do they just lie around and play "bed games" etc.
on the other hand if you have /encourage / allow they could work on keeping current skills more or less in use, and possibly do basic training of other skills if they can come up with an example that is plausible for how they are trying to maintain or improve a current skill/ability.
lets say the player gets it in their head that they want to improve a chars strength obviously the best way to do this is routine weight lifting
endurance would likely involve running and carrying stuff
reflexes or agility would involve acrobatics or gymnastics type exercises and the like.
a lot of the time I made my players tell me, and and/or suggest things that the character would likely be doing to "justify" a skill or stat upgrade.