Physisists seem to be divided in two layers: those who claim that vertices in Feynman diagrams contribute +ieγμ and those who say −ieγμ. As an example, Griffiths says + and Schwartz says −. Quick Googling seems to indicate that the two layers are of the same size.
The difference could easily be due to the sign convention for e: do we have e>0 or e<0? Unfortunately, neither Griffiths nor Schwartz seem to reveal their respective sign conventions for e.
In general, sources tend to be silent about their sign convention of e. An exception is Kleinert who states e<0 on Page 803 right after Equation (12.10) and says that vertices contribute −eγμ on Page 814 Equation (12.96).
My question is: does everyone agree that a vertex contributes +ieγμ if e>0 and −ieγμ if e<0?
References:
Griffiths: "Introduction to Elementary Particles"
Schwartz: "Quantum Field Theory and the Standard Model".
Kleinter: "Particles and Quantum Fields" / http://users.physik.fu-berlin.de/~kleinert/b6/psfiles/Chapter-11-qed.pdf