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I have an 18 speed and want to swap the wheels out, but the new rear wheel has an 8-sprocket cassette and the one on the bike has a 6-sprocket . I plan on swapping them but just for fun I tested it out without swapping them and the 8-sprocket rides just fine even though two of the sprockets will never be used.
Would it cause problems to ride with the 8-sprocket or can I just leave it on and won't experience any differences?
I plan on swapping them ...
I'm afraid you may be disappointed to discover that the six-speed sprocket on the old wheel is a "thread-on freewheel", whereas the eight-speed one is a "casette". In that case, the two systems not compatible; the swap is not possible. Moreover, different tools are required to unfasten them.
the 8-sprocket rides just fine even though two of the sprockets will never be used.
So you might think, but six speed freewheels and 8 speed cassettes do not use the same cog spacing. Your six speed shifter's positions do not correspond neatly to six out of the eight cogs. See the casette/freewheel spacing table on Sheldon Brown site. E.g. Shimano 8 speed cassettes use a 4.8 mm spacing, whereas "'Regular' 6 speed" is 5.5 mm. This could cause problems with quality of shifting. You will notice when you start putting down mileage.
As long as you're running a narrow six speed chain, there shouldn't be any issues as the width on those is the same as a seven/eight speed chain. You may want to adjust the rear derailleur and you can probably hit all eight speeds on your cluster. If you have index shifting you will need an eight speed shifter.
