Physisists seem to be divided in two layers: those who claim that vertices in Feynman diagrams contribute $ + i e \gamma ^ \mu $ and those who say $ - i e \gamma ^ \mu $. 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 \lt 0 $ on Page 803 right after Equation (12.10) and says that vertices contribute $ - e \gamma ^ \mu $ on Page 814 Equation (12.96).
My question is: does everyone agree that a vertex contributes $ + i e \gamma ^ \mu $ if $ e > 0 $ and $ - i e \gamma ^ \mu $ if $ e < 0 $?
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