I am reading articles about css. I found out that many of writers suggest to use % value for div\'s width or height.
I am using pixels all the time.
Why should I
Because if it's percentage based content flows making it easier to implement a responsive site (easier = (less (time = money))).
Drop the relevant floats, sort out your menus and and anything else after dropping in some media queries and you've got a site working on different resolutions and devices.
I use a fluid grid system using percentages and a ems for text sizes. Pixels are easier to work with but not in responsive terms. I do wrap my size in max width container.
PS
Remember though everything boils down to pixels in the end.
It's not clear which context you are referring to (what type of elements). However, you need to be careful when using pixels for font sizes.
I generally use pixels for divs to keep my layout as expected. But I avoid pixels for fonts. Different users may want the font at different sizes. Why take that away.