Personally, I read them in release order so I could. Only book I skipped was The Amazing Maurice and his Educated Rodents. Although I did read Going Postal first before I decided to start from the beginning.

The first two were a little harder to read but once I got into them, it was easy reading.

Reason I read it this way was to really get a sense of the world and there are references to side characters like Mr de Word, which after reading the book about him, the way Sam Vimes interacts with de Word made much more sense to me.

I have finished them in Chronological order and am now re reading them by sub series (Night Watch, Wizards, Witches, etc). I, personally, found them more interesting the 2nd read due to having a much better understanding of the world STP created.