Whatever the case, make sure to invoice your clients consistently. You're more likely to get paid on time when your client receives invoices regularly on the first day of each month, than getting them sporadically.
Accurate billing is extremely important when it comes to independent contractors and small business owners. You can't afford to lose a customer over something that should be done right the first time. Business owners who provide detailed, prompt invoices straight to the customer's inbox come out way ahead in the credibility stakes.