Easy lads!!
Basically, I read in the book 'exercising the penis' that stretching before jelqing makes the penis more susceptible to its effects. This makes sense, the penis is more stretched and malleable and so in some way (that is a mystery to me) the constant squeezing of blood will probably have a greater affect than if it was being squezzed into a less ...worked penis??
So anyway, not sure about the science of any of this stuff, but stretching before jelqing probably does enhance its effects but to be honest, if you warm up right then stretch and then later on warm up well and jelq, providing you stretch and jelq effectively I think you will still gain but just maybe slightly less than you would have if they were combined - but it should still work!!
Do what is best for you mate!!