Honestly, any thin layer that won't wash out is more of a cosmetic than a functional issue in my opinion.
A layer heavy enough that you can't check the coolant level without removing the cap could be enough of a PITA that I'd consider replacing it, since in the Focus the reservoir is under pressure (as opposed to an overflow tank that isn't) and you don't want to have to wait for the car to cool B4 checking levels or regularly remove the cap while still pressurized. (somewhat hazardous)
My old P/U for example, has an overflow tank type system - so even though the bottle is a bit hard to see through by now (age & crud) I can always peek in the top since it isn't pressurized.
$ to spare, a new bottle would be nice! it just isn't absolutely necessary.