Again, not wishing to rake up the bunfight if you can charge more and keep your customers happy then I'd suggest it's a good idea. If your customers are keen on price, you might lose more than you'd gain. If not, you'll be making more.
In my experience they are happy with the reasoning I give them, namely that a 6 weekly (basic period in my case) keeps the windows clean, when it gets to 12 weekly, I'm having to clean off dirt, so they have to pay for that. Regardless of what we think about that argument, my customers are happy with it.
Vin