I have read them in publication order (because I read the books when they were published
). But as others have said, nearly all books can be read stand-alone. But of course it makes sense to read the subseries beginning with the first book. If you like some subseries, no problem to read just that. Maybe one could argue, that the later books playing in ankh-morpok should be read in publication order, because they all intertwine. And as was also mentioned, there are some totally stand-alone books, some of them are among Pratchetts best: Small gods, The Truth, Monstrous regiment, Pyramids. Oh, and of course all the first books of the subseries are great, that's why they became series
So reading Wyrd Sisters, Guards! Guards!, Mort and the first book makes sense, to get a taste of the different series.