to be honest mate, that is one of my biggest concerns about doing a round lol 
fingers crossed, how much as a percentage do would you say has to be written off untill the following month? or do you deal with it in a different maner when strugling to get payment asap?
I take it you're asking about cash flow and getting your dosh?
Most of the rich established guys here will shout 'evelopes', as in you clean and leave a SAE with payment instructions.
I do this also, but only for my rural accounts. I have two estate areas I collect from on a Saturday morning; I find arriving there about 10.00 am works for me and it takes an hour; twice a month; I can park up and walk round. If they're not in, I leave a SAE with payment instructions.
If I don't get sent a cheque, I just add it onto the following month.
But if you're new, with loads of drive betweens, collecting is going to be a pain.