[greglepore]

I'd change the water on arrival only because its been sitting unattended and might be funky. Assuming you're not getting temps well below freezing you'll be fine just letting it sit. After arrival, drain, refill, adjust water chemistry (usually just add some bromine) and your good to go.
The one thing that makes the water funky quickly in use is cosmetics. They're oil based and don't degrade. There are clairifiers that bond to them but its easier just to avoid getting them in the tub.

If there are long times where you're not there, if its not going below freezing you can just turn it off and deal with it upon return. Shock it with bromine and algicide before you leave, and set the filter to run for an hour a day or not at all, temp off. Or drain. Up to you.