Certain operations will destroy any existing UV work. Aside from that, when you apply UVs on a lowres model and then smooth the model, you get texture distortion because UVs are subdivided in a linear fashion even though most of the time you'd want your model smoothed using a non-linear method.
Ideally, it's more efficient to UV the lowres model and then smooth it. But I find that most of the time doing all of my modeling in lowres and then simply applying a smooth doesn't do much for the look of the model. Usually after doing a significant amount of modeling on the lowres version I do some edits on the hires version to finish the model off. Having UVs layed out at this point would give you distorted textures without further UV work.
There are exceptions but in general I find it best to do my UVs last for three reasons. One, I want to model freely and efficiently without concern for my UVs. Two, I then know my UVs are as close to 1 to 1 as I get. Three, I'm comfortable enough to UV anything efficiently at any res. It really depends on how you go about achieving your final result. Sometimes you can salvage UVs shells, sometimes you have to generate them from scratch or rebuild them. As you work with UVs more, you'll be able to keep track of your UVs without even looking at them.
'cel