I remember reading threads in the past about difficulty shfting into 5th gear, with no real resolution.  I periodically have an issue shifting into 5th - either it stays in 4th or just goes neutral.  On the flipside, I have the occasional problem getting out of 5th.  Just the other day, I was cruising in 5th and went to downshift to turn, and the shfter was just all floppy and wouldn't go into any gear!    I had to roll off the side of the road and kick it up and down until it finally caught.  Then it was fine the rest of the ride.

So is there any adjustment or fix, or is this just the way it is, so we deal with it?  

If it has whits or teels... you're gonna have trouble with it.

If it's only an occassional shift, I wouldn't worry about it too much.
If it occurred every ride, I probably would.

verify there are no binding issues with the linkage or levers.
check your adjustment, the lever or the spline is about vertical and the toe spud doesn't rest on your boot inbetween shifts.

a few have had to open up that cover and fiddle with the shift mech due to something getting out of place, rare.