There will be times when you know certain invoices aren't going to be paid. This is called bad debt, and you can write off your invoices to a bad debt account to record this. Due to the extensive nature of the steps required to set this up, I'm going to provide the article below. This will guide you step-by-step on how to set up a bad debt account in your chart of accounts, how to create a bad debt item, creating a credit memo for the bad debt, and applying that credit memo to the invoice.
Unpaid invoices have no impact on cash reporting, so whether you write them off to bad debt or try to get paid is immaterial to cash basis reporting.
How you report your company for income taxes, may or may not have anything to do with sales taxes. Almost all states required that you report and pay sales taxes on the accrual basis. You need to check with your state comptroller for sales tax to find out.