The only times you need to protect your lens is when you go out into conditions where there is a very real chance of damage to the lens (riots, sandstorm).
Given the price of this lens, I'd consider it a throwaway under such conditions. If it gets damaged, just buy another one. Were it a Nikkor 80-400 VR (€1700) I'd say get a B+W KR.1A for it at €75 or so, for a €300 lens such a filter is overkill if you want it only as protection.
At f/8 or up you can use the hood over the entire zoomrange, which protects against people bumping into you. A filter won't protect you from sand getting into the mechanism during a sandstorm, and it's unlikely you'll be using it in a riot where someone could throw a bucket of paint over it.
I have a filter on an expensive pro grade 28-70 lens, but that's mainly because the filter slightly warms the pictures as well and that lens is exceptionally cool (iow, I just like the effect of the filter). And for that one I did get an expensive filter of course, having seen the negative effects on picture clarity cheap filters produce (I now keep cheap filters only as emergency replacements for lenscaps in case I loose those).
I wish I were flying