As someone stated it before, it all depends on the wah you use; some years ago, I stuggled to find a wah which could cope with distortion and fuzz placed after it, which could bring expressivness to my tone. My quest has ceased as soon as I tried a Geoffrey Teese RMC 6 WoF wah and I still use after 10 years.
As a matter of fact I'm now gassing a second wah, a RMC 10.
To my hears, a wah placed after the dirt usually mud your tone, bluring definition and harmonics, and the effect is far from being subtle . But it's a matter of taste.