Sup EG

I have an all stock Fender Lite Ash Strat (2 pin trem) which is having some tuning stability issues. It sometimes progressively detunes itself while I'm playing...and especially when I use the trem for a bit. Usual suspects: G string mostly, sometime the B, even had the D once. Some days are better than others so it's really frustrating.
So I'm thinking locking tuners. How much will these help? I mostly use the bridge for light vibrato effect, a small amount of lifting the bar up (barely does a semitone). Extreme dips...sometimes, but hardly. This is my only guitar without a fixed bridge so I feel a little bit out of my depth on this one.

try getting a good setup first. make sure the tech pays attention to the nut as that s usually the culprit. personally I hae my strat trems set to float so you can pull up just a tad on them. this often unkinks errant strings. locking tuners are a good thing as well.
I second getting a good setup. The nut slots may not be wide enough or finished properly on the bottom of the slot, which can cause binding and erratic tuning. This is especially true if you use larger gauge strings than the factory setup. A set of locking tuners is a worthy investment, especially if you use the vibrato often. I tend to use Fender Bullets on my Strat, as they seem to stay in tune better than my usual D'Addarios, though this is not true for everybody. Also, make sure the breakover points on your saddles are well lubed, as well as the fulcrum points on the bridge and the nut slots. One of those tiny bottoms of PTFE or graphite lube will last for years and fits easily in the pocket or storage compartment of your case.