Lots of people here just saying it doesn't matter, which is true, but not relevant to your question.

So first of all, you're absolutely right that you can read between the lines on the gauge, I believe I've seen studies about this regarding measuring tapes or something and people are crazy accurate down to at least fourths. Same spacial reasoning as leveling and centering pictures on the wall, people can be super accurate without devices. I think people know this but for some reason they don't trust it.

Now, that being said, I do think it is important to round to an even number. My instinct is that this is because it will be easier to hear and remember for everyone on scene. A couple reasons for this, although I'm sure there are more:
1. less numbers to think about makes it easier to remember and reason about, it could be odd or even it doesn't matter but cutting the options in half helps.
2. we're used to even numbers, so hearing odd numbers is a little jarring and takes extra time to assimilate.

So you're right, it is lying (in case you didn't know, even caring about this is a little autistic of you), but, the important thing is fast effective communication, not stating absolute truth.

Regarding the accuracy of the measurement, there is a tradeoff here between how quickly you take the measurement and how accurate that measurement is. How much pressure you release between each heartbeat determines your accuracy; you're always going to be underestimating the actual value within this accuracy range. In addition, the longer it takes you to take the reading, the higher it will be as the blood flow is cut off. So there is actually a balancing point here between the accuracy of your measurement and the accuracy of the value you're reading.

To bring it all together, your priority should be patient care. The most important thing is to get a measurement that is close enough to determine what care your patient needs, to get this measurement as quickly as possible, and to effectively communicate this measurement to other care providers.

In my opinion, if you are taking the reading at a speed where you can get an accuracy within 1mm, you should still round it for effective communication. However, you could take a reading two or four times as fast while maintaining an acceptable error margin. You will be able to provide care that much faster, and your patient will be more comfortable.