In deed you would if there was one; error that is. That is why 2 FMC's are needed for oceanic crossings. They have been tested to a reliability of 1 in millions, and designed to tell you if they disagree with one another. That was in simple IRS days. Now with GPS the likelihood of error has reduced to 1 in squillions.
But you plot to position on the chart relative to your route and WPT that has already been plotted. If the FMC says you are en-route the 99.999999999% chance is you are. If it says you are off-route then the head scratching starts. So you are doing more than just plotting; you are comparing where the FMC says you are to where you should be.
There will be some old grey hairs who cry, "being back the sextant", and the reply will be, "who's that? A sexy relative?"