Pretty much anything below 1/32 (0.8mm) of an inch, we'll switch to decimals. 0.0001 inch is valid with no common way to make that neater. No such thing as 1 mili-inch.

.001" is a thou or mil (1/1000 of an inch). That is commonly understood in any industry that requires that precision and also doesn't already work in metric by default. 0.0001 would be 0.1 thou, but honestly any time I've ever seen anybody need more precision than a whole number thou, they worked in microns or nanometers.