It is not a huge deal, but may have some effect on the permanence of your photographs. The film won't care, unless you have over-used the fixer; then the fixer won't be able to do its job.
The purpose of the fixer is to convert unexposed silver salts in the paper or film into compounds that are water soluble and can be washed away. BUT with fiber based paper (the nice stuff -- I have a bias against resin-coated papers) the fibers also absorb and bond with the fixer and it can be difficult to wash away. With film it doesn't matter so much, because the film base (polyester) is not absorbent. So you want the paper you are fixing to be in the fixer the minimum amount of time to adequately convert your unexposed silver. In other words, you want to fix your prints "just enough." More is not better.
I suppose it is possible that if the fixer is too strong, and you fixed for too long you could also degrade the image-forming silver, but I've never heard anyone complain about that.
So: Using fixer at the higher concentration should not be that big a deal, but if you are fixing at that concentration for an amount of time meant for the lower concentration, it may be harder to wash.
BTW, if you are mixing fresh fixer for each processing session, it will also not be that big a deal. If you use fixer over and over, eventually it will become exhausted (that is, it will contain too much silver from your prints and film for it to work).
Hope this helps.
Ben
P.S. Fixer is a pretty expensive component of the developing process. Using it at high concentration will cost more.
