is strength an important attribute for a WR or is not so much?

If you want him to block and he otherwise has a good blocking rating.

Or you want to move him to another position sometime down the road

Reviving this. Is strength useful for anything other than blocking? Attributes page doesn't make it seem so.

From what I've seen, it helps create space off the block. Don't know if that's confirmed.

I should have specified that before. I meant blocking in terms of both offense and a defender separating from or pushing an offensive player engaging in a block.

From what I've seen yes. Strength is a big factor in blocking and powering through blocks.

That's what it says in the getting started guide. I want to know if it is used for anything else. Otherwise, I can weight strength really low on RBs since they aren't required to block in my system. I can also rate it low for all of my secondary players since they won't be engaging in blocks with much frequency.

You might want to reconsider with respect to the secondary. I think they use strength to get away from blockers (and make a tackle).