It does depend on the price you are charging too, if it's nice and high then don't worry too much about it, if the price is on the low side anyway then you can use it as an excuse to put the price, using the argument that after 8 weeks the work is harder, and therefore you have to charge more.
I have plenty who are 8 weekly, I no longer do repeat cleans above that (not domestic anyway), it's 4 weekly or 8 weekly, end of story.
8 weekly or 4 weekly, not often there is the remotest difference in the work involved, I have never found it makes a lick of difference to be honest, but you can make out it is a reason to slap in a price rise.
Ian