If it's a wet recipe (casserole, pot roast) you're more likely to find the meat falling apart than tough. 

Medium is non standard and there's actually quite a bit of variation between slow cookers. Medium on our current model is pretty much the same as high on our old one; high on the new one is a bit fierce for most recipes without adaptation. 

I suggest that you cook it on high, but if you're around when it reaches the minimum cooking time, then it down to low (not warm) until you're ready for it.