The best thing about using an invoice system is that you can track your time against the amount you're getting paid for each task. In order to run a successful business, you want your hourly rate to be profitable so you're not losing time and money. Another benefit of using an invoicing system is that you'll never save over an invoice and lose the work you've calculated for another client.
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.