To clarify:

* Jana and Jamie are correct and I was wrong -- it is vitally important for user trust that the value users see is always the value that was given to us. IF, that is, the units they are looking at are the same units they saved it with. If you upload your data in mmol/L, and then view it in mg/dL, it obviously won't be.
* The important part of this discussion is that we must store readings in floating point, not in integers, and we must store them without truncation or rounding. This way we can reliably convert them between formats without loss of information. Floating point numbers have all the precision we need if we use them correctly.
* With respect to storing multiple units in the database, that is a very bad idea. You then put the burden on every client and every user of the database to do conversions themselves -- and they may not be so careful (the prime example is the 18.018018 thing). It's a prime location for future bugs that may not even be our own bugs. We really must store the data as a number, and we must store it in some unit that makes sense. *It really doesn't matter which one* as long as we store it accurately in floating point. 
* Asking what other people do won't hurt, but frankly, it doesn't matter much unless we also know why they did it that way and whether they did their math right. I'm afraid I don't have a lot of confidence about that.
* We can store the original string that we get from the source data -- which, by the way, could theoretically be a number from a sensor in a completely different form -- so that we can show it as part of provenance. But the data that will be read and aggregated by the system will be a floating point number and all the rows in the database will use the same units.
* I say all of this with confidence because I wrote some python code that demonstrates it all: https://gist.github.com/kentquirk/3644a03c6a037b7cf0ba

There's nothing magic about the units we use in the database -- this is only a controversy because we first proposed a model that would have lost precision. That was a poor design that we are changing.