I've never had a problem going for a wander around the plane. Might be best to avoid it while they are still serving food/got the trollies out. Perhaps mention it to the crew if passing them that you want to stretch your legs. But not sure why you'd want to use the economy loos? They have a lot more traffic passing through them so are likely to be dirtier/have a queue. Personally I'd go for the walk and use the higher class loos.