I agree with Tim; if they haven't reached your rad. saturation limits, Give it an added bit of insurance.

Amifostine is an option, although if you've already had radiation previously, you're likely not going to get a full course of aggressive radiation this time around. The parotid has a permanent damage threshold of somewhere around 6000 units of direct exposure. Below that and you may retain partial function at least on one side.

I guess by my thinking if the option is having limited/no saliva and having to use a water bottle constantly compared to it not mattering whether you had saliva or not, I'd go for needing the water bottle.
Good luck with whatever you decide
Wayne