Sales tax is an amount added to a sale, collected from the customer, and forwarded to a taxing authority.
It is not revenue
The business is a collection agent. The money belongs to the authority from the moment it is charged.
This is the whole of the risk. Sales tax sits in the same bank account as everything else and looks like money the business has. It is spent by accident, and the shortfall appears at filing.
Judging what is available from the bank balance produces exactly this error — see real cash balance.
What is taxable varies
Rules differ by state and frequently by locality, and service work is treated inconsistently: taxable in some jurisdictions, exempt in others, and in several, taxable only when materials are supplied with it.
A contractor may also owe tax on materials they consume rather than resell, which is what use tax covers.
Because the rules are local and change, this is a question for an accountant in the jurisdiction rather than a general answer.
Filing
Returns are due on a schedule set when the business registers, commonly monthly or quarterly, and are due whether or not any tax was collected in the period.
Penalties attach to the filing as well as the payment, so a return filed late with nothing owed can still cost money.
Holding it separately
The reliable approach is to move collected tax out of the operating balance as it is collected, the same discipline as a tax reserve.