Jordan,
It's definitely worth the investment to get the 2nd set of carts just to store the flush solution. You can then keep these carts filled with flush for cleaning out the printer as well for loading the printer with flush solution in case you plan to leave it idle for more than a week. Storing your printer this way will keep your ink lines and print head healthy and free from permanent clogs which can develop.
It might seem expensive to get the carts now, but it's going to be cheaper than purchasing a new print head or even a new printer if your current one suffers from a catastrophic clog.
Nick Enghardt
St. Edward's University Advanced Imaging Lab Manager