You can now round the total of an invoice, an estimate or a recurring invoice, and the difference shows on the document as a line of its own. It helps most when your customers pay in cash, because several countries require cash payments to be rounded to 5 cents.
While you work on an invoice, click the totals below the line items. That opens Invoice details straight on Round the total to:, which offers Nearest 0.05, Nearest 0.10, Nearest 1.00 and Custom amount. The same panel opens when you click the invoice number and dates at the top of the document. The total changes as soon as you choose, and a Rounding line with the difference appears above the amount to pay. A total of 10.03, for example, becomes 10.05 when rounded to the nearest 0.05, with 0.02 on the rounding line.

With Custom amount you type the difference yourself, up to 5.00 in either direction. You need it when you have agreed on a round 100.00 with a customer, for example, and the VAT calculation lands on 100.01.
If you round every invoice the same way, you can set it once. Open Business settings and choose a value for Round invoice totals to in the section Invoice late fees, payment terms & notes. Estimates have their own setting, Round estimate totals to, in the section Estimate expiration & notes. New documents start with that choice, and you can still change it on each one.
Belgium, Estonia, Finland, Ireland, Italy, Lithuania, the Netherlands and Slovakia require cash payments to be rounded to the nearest 5 cents. If your business is in one of them, Nearest 0.05 is marked as suggested on invoices in euros. The Payment method is on the same panel, and when you choose Cash there, that rounding turns on for the invoice, unless you have already picked another one.
Bank transfers are always paid to the exact amount, so nothing is rounded until you choose it.
The rounding line prints on the PDF and is carried on e-invoices too, so your customer's accounting software reads the same rounded amount. An estimate keeps its rounding when you turn it into an invoice, and a recurring invoice passes its rounding on to every invoice it generates. On a credit note the rounding has the opposite sign, so the invoice and its credit note still cancel out.