I have the impression you don’t know how rounding works. The two temperatures you quote would be different even with only full numbers.
In any case, for the case of body temperature, you do exactly that, use as many positions after the comma as you want. (Usually more the one is pointless though as you can reliably measure that anyway)
Until they have a fever, then 38.1 and 38.9 can be significant.
I have the impression you don’t know how rounding works. The two temperatures you quote would be different even with only full numbers.
In any case, for the case of body temperature, you do exactly that, use as many positions after the comma as you want. (Usually more the one is pointless though as you can reliably measure that anyway)