There's no point saving them up as you only have 20 trades, with a max of 2 per round. So if you used up 2 every week for the first 20 weeks, its only the last 2 weeks that you would go without. Conversely, you may have saved, for example, 10 trades in the last 2 weeks, but you can only use 2 per week, so they're wasted.