I think the most important thing is that the accounting is done correctly. You'd want to be careful to show that your cash received is against future revenue (otherwise your gross margins will look too good, as presumably you still haven't incurred full cost of sales yet). It's down to accounting policies. (Some companies we deal with - particularly software, will not recognise revenue and won't issue invoices until their terms & conditions are signed - regardless of payments made).
Maybe ask your accountant. (I am not an accountant, by the way... above commentary just based on business experience)