Personally, I don't think there is such a thing as a bad starting point with Pratchett. While some books might spoil some others, most are pretty stand-alone and don't require much pre-knowlege to figure out what is going on.
If you do want a starting point, there are a few because most books tend to focus on a particular set of characters and there is a starting poing for those particular characters.
Rincewind - The Colour Of Magic (which even Terry himself doesn't recommend people start with.)
Vimes and the Watch - Guards! Guards!
The Witches - Equal Rites
Death - Mort
Moist - Going Postal
Then there is "Small Gods" and "Pyramids", which are pretty much standalone.
BTW, here is a link to a decent visual guide to the books and what the order is.
http://www.lspace.org/books/reading-order-guides/
Jason