| It's really the change in temperature that matters. So if you keep your bass in the basement, and the basement is colder in winter, there's a good chance you'll have to adjust your truss rod and maybe intonation as the seasons change. But once the temperature has settled into whatever it's going to be, your bass should stop changing too. Stable temperature = stable bass, changing temperature = changing bass.
For that matter moving the bass from one place in the house to another can do the same thing if the different places are at different temperatures. And some basses are more stable than others, too, so it depends.
And in my opinion a gig bag or case won't help if you're leaving the bass in your basement for more than a few hours. If you're taking the bass from somewhere warm to somewhere cold a bag or case may help keep the bass warm a little longer, but over the course of several hours the bass will still get cold. Unlike you or I wearing a coat, the bass doesn't generate its own heat so it will eventually be the same temperature as its surroundings, even with a bag or case. If the bass primarily stays in the bassment just leave it out and let it acclimate to the conditions, and then adjust if needed. | 
