You don’t need to worry; everything works like intendend with 18" wheels 
More likely you get in trouble if you want to use wider tyres than usual on 16" wheels.
Please jump over here to read why I’d prefer 18":
Re: EVO or SX for general aviation pilot
I believe it is better to “waste” time during learning new things (just for example the tightening of one or another loose spoke*) than to waste time and force each time riding (by deforming flexible plastic wheels).
*All you need is a spoke wrench, a small piece of wood (cooking spoon, piece of branch) and your ears 