There are a few options. The first would be to turn off email bills for those clients that will not get bills emailed. Since you said it varies month to month, that may not be practical. 

The next option would be to put all invoices in proof. Then approval only those that will be emailed. Send them out and then approve the rest and uncheck the email boxes when it asks to email. 

The third option is to approve all invoices. When the email to list opens you can uncheck the ones that you don't want sent out.