Spend the time and get them right now, use something like Gtechniq C5 wheel armour on them once they are clean and you will never have the same problem again. My wheels will pressure wash clean most of the time, the brake dust just can't take hold on this stuff.
Get yourself a large EZ Detail wheel cleaning brush and add cleaning the wheel inners to your cleaning routine every time you wash the car - just with normal wash shampoo, you won't need anything else - takes a few minutes but your wheels will always be immaculate.