If you are welding to a code like AWS D1.1, unless the 7018 came out of
a hermetically sealed container, you have to dry it before using it.
The boxes at Home Depot are not hermetically sealed. The flux will
absorb moisture from the atmosphere even though there is plastic wrap
around the box. The moisture will result in increased probability of
porosity, hydrogen embrittlement, etc. in the weld. If you aren't
welding to code, it is up to you. It may or may not make a significant
difference in the weld. I would consider it unwise to ignore the
manufacturer's instructions welding anything to which liability might
attach, like a trailer hitch. FWIW I have seen successful bend tests
done on welds made with 7018 that came of non-hermetically sealed
containers of dubious history.