It is quite common for a bass to go out of tune. You shouldn’t play with your bass out of tune. An out of tune instrument will ruin the sound of the entire band and throw the singer’s pitch off. So, ideally you should check that you’re in tune every time you play.
So what makes the bass go out of tune?
The simplest cause is the tuning key moved! It might have been bumped especially if you have a soft case. Someone else may have been playing with your bass. Little kids love to play with those shiny tuning keys.
The next most common cause is weather conditions. As temperatures fluctuate, your bass expands and contracts. This causes the neck to shift and the bass goes out of tune. Another big factor is humidity. Your bass is a big chunk of wood. That wood is constantly absorbing and releasing water in the air. Again, this causes everything to expand and contract making your bass go out of tune.
Sometimes putting the string on incorrectly will cause tuning issues. Make sure you read the how to string a bass article. If you have the string wrapping over itself on the tuning post, the string may shift around more.
Less commonly, the nut may be catching the windings of your strings. This can happen if you change to a heavier gauge of strings and the nut is not cut to accommodate them. If you plan on sticking with a heavier string gauge you should get your nut slotted properly for them.
Another less common problem is the bass tuning key may be bad and can’t hold the string to pitch. If one string in particular is always going out of tune, this may be the problem. Tuning keys are fairly easy to replace on your own.