Indeed, but only a preference. If using both cogs on a double sided fixed hub increases your enjoyment or success, then go for it. It is even theoretically possible to use a double chainring with a suitably different pair of cogs to maintain chain tension, but a high profile fixer (no longer, apparently, OTP) tried it some considerable time ago, only to find a greater frequency of chain breakages, which he took as a hint not to buck the system.