There are four different reasons to upgrade that I can see.

1) For looks. If you are trying to recreate a vintage style guitar/bass, then the stock generic kit tuners aren't going to do it.

2) For operation (1). If you are a heavy trem user, then you'd probably benefit from locking tuners.

3) For operation (2). The stock kit tuners are often quite low geared, some with a 12:1 ratio (though the only set of kit tuners I have handy are 15:1). The higher the ratio, the easier it is to get the guitar tuned, as each turn means a smaller movement of the string. As you can now get guitar tuners with 18:1 ratios, then you may want to upgrade for that reason. There is also less backlash in the gears. Once you get to 12:1 and above, there's enough resistance to stop the string tension pulling the posts round, so there's no need to upgrade for that reason.

4) For operation (3). The stock tuners are of varying quality. You may get a set where they are all fine, or you may get some where either they are very hard to turn or the peg is very loose so an upgrade is a good idea. A good set of tuners should have a consistent smooth feel with no graunchy bits and minimal backlash in the gears.

With the cost of upgrade Grover tuners from Pitbull being so cheap (for guitars at least), as long as you can get a set of tuners that fit the looks you want, then for me it would be an easy decision to upgrade them each time.