Xero can handle four decimal places in the Unit Amount and Unit Price field. Xero can also handle two decimal places in the percentage field (i.e. 12.75%). (more details from Xero)
Xero rounds everything to two decimal places for the line totals. Tax is calculated at two decimal places. And subtotals and totals are stored as two decimal places. (how Xero rounds)
Salesforce can handle up to 8 decimal places on Opportunity Products price field and price book field, but only if you file a case. (more details from Salesforce). This is further complicated in Salesforce as they do not allow more than 2 decimal places in the Quantity or Percentage field. Additionally, the Amount in the Opportunity field only allows for 2 decimal places.
Salesforce discounts are two decimal places, the same as Xero.

Breadwinner supports both two and four Decimals on Line Items. By default, we show up the four decimals on the detail page of Line Item.

Creating Invoices in Xero with Breadwinner

If you are creating an invoice in Xero through Breadwinner, then we will allow you to put any number of decimal places in the fields. However, only 4 decimal places are allowed by Xero, so the fifth decimal place onwards will be dropped when we pass the invoice to Xero - and for the percentage, the third decimal place will be dropped by Xero.
Please note - this is only for the line items Quantity, Price, and Percent. Xero calculates the line item totals, as well as the subtotal, tax, total, and Xero calculates this at two decimal places (this is beyond our control).
So, Breadwinner allows you to enter the full decimal places into your Xero Invoice Line Items.

Invoices in Salesforce

Breadwinner keeps a copy of all of your Xero Invoices in Salesforce. These fields are 16,2 fields, meaning that you can have 16 numbers before the decimal place, and 2 numbers after the decimal place. This means that all data from Xero is saved in our fields, as Xero only calculates 2 decimal places on subtotal and total (and related) fields.
Of course, the 16 character limitation before the decimal place limits you to Invoices with a Total of just under 10 Quadrillion Dollars. If you are indeed billing in the Quadrillions, we have a special pricing plan for you - contact our billing department!