OK the glider is a Skytype Mk1, there are no thermals etc., the best glide speed is quoted as 60 knots, you fly either 5 knots faster or slower than this.
Does the 5 knots either way increase or decrease your ability to get back to base for a landing assuming that the distance is at the optimum?
Assuming still air and nil wind, yes. Both decreas the distance you will fly through the air before the ground rises up to meet you, and as it's nil wind it will have the same effect on your distance over the ground.
5kt either way will not make much difference in a modern glider - perhaps only a field or six short of your destination, if that bothers you.
However, head and tail wind
do make a difference. Into a head wind you want to fly somewhat faster than best L/D (the easiest way to understand this in your example is to imagine a 60kt headwind - then best L/D achieves no distance over the ground whilst flying faster makes at least some progress), into a tail wind somewhat slower but never slower than minimum sink (for your example glider, probably 45-50 kt).
Calculating these speeds precisely requires some form of glide computer. As a rule of thumb, adding 1/2 headwind to best L/D gets you fairly close. Sod's law means you will never have a tailwind on a marginal final glide.
As you approach the airfield, whether it is rising up the canopy or sinking is the best guide - but if it's rising you're already so low that you'd better have picked your field already.