In a comment to The kerning option of microtype Karl Karlsson writes:
Most of the typographers agree, that Font expansion distorts the font. Personally, I see it, and I agree - the distortion is quite visible for the trained reader.
I've always thought font expansion was great because it reduced hyphenation and I haven't noticed any distortion. The reason that I haven't noticed any distortion is probably because I've read documents on screen or draft/economy mode inkjet prints. When I recently got a document printed on a laser printer I noticed the distortion and it was irritating. The font I noticed it for is Latin Modern in 12 pt with the default font expansion settings. Before I only had documents printed on laser printer as two pages per paper (same font and font size) and then I did not notice any distortion.
In the microtype manual I read that the amount of font expansion can be configured by changing stretchability and shrinkability of a font via
shrink and that their default value is 20 (§3.3). My question is: what is the greatest value for font expansion for which distortion is not noticeable when printing documents on a laser printer? I haven't got a laser printer so it's hard for me to test different values. Also, I'd appreciate any information on fonts for which distortion due to font expansion is rarely noticeable.