On a high pressure gas shock, the (nitrogen) gas and oil are separated. The piston can be on the top or bottom, or even side ways, it makes no different. Clearance usually determines which way it's mounted since a reservoir is bigger round than the piston. (some shocks have external reservoirs, like in racing - which gives you more choices on mounting them). Monotube shocks have more dampening and less heat - which is why they are used in racing and performance applications.

Older style twin tube shocks the oil and gas are mixed (which causes cavitation - aeration - like foam - not good) to they have to be mounted a specific way.

On a model it won't matter - unless you are building a specific car / truck and want it to be exactly like the real thing visually