Yes, there is a huge disparity between costs at each petrol station, and cost savings can be significant if you choose the right ones. More remote stations are more expensive, and I hate to say it, but independently owned stations are normally much more expensive. That's not to say all of them price hike; many family-owned businesses should be supported as they offer essential services on the NC500.
Supermarket fuel stations are generally cheaper and less prone to price changes between stores, but even these sometimes take advantage and hike prices.
Generally, you will find the best prices at the start of your trip in Inverness, so leave with a full tank for the best value for money.