It's not a big deal on the "rounding" since - based on past experience - I put in a column for "adjustment to principal" and one for "adjustment to escrow." About every 7 or 8 months I seem to add a penny, and then take it back off about 2 months later.
It's quite obvious that their calculation "carries" fractional cents that don't print on the statements. When they "report" .04 as the "current balance" they may actually carry over and use .03925... as the starting point of their next months calculation. The key to "absolute accuracy" would be to know how many significant figures they carry. If it mattered, I could run trial balances to "discover" their computer resolution, but it's not a priority. I've found that the last people likely to be able to "just tell me" would be anyone in an accounting department - "It's magic, the computer does it."
The only reason for "calculating" my record of the mortgage in my own spreadsheet is that in Excel it's very easy to do, and I've had experience with mortgage companies that did make a whole lot of rather large "errors." I'm reasonably confident, by now, that my current company is at least "commercial grade honest," which was my original concern when I set up the sheet.