Petrol is fine stored in a container designed for the storage of petrol (i.e., a petrol can!). Unleaded does 'go off'; if it's left in a mower for any length of time unused (say, over winter) it can gum up the float chamber or stick the diaphragms together (depending on the type of carburettor). Drain the tank at the end of the season and run the engine until it stalls so there's no petrol left in the carb. If a 5l can lasts you a long time, there are additives you can buy to stop the petrol deteriorating.