It all depends on how much you trust your mechanic. If my mechanic told me my tranny needed service, I would leave it right there and let him service it. For the injectors, go down to PepBoys or any other autopart store and buy some fuel system cleaner and follow the instructions on the bottle.
Does your tranny feel strange to you? Is it making noise it shouldnt? Leaking fluid? Grinding? If it is any of these things, please let it get serviced, or you could end up with a bigger problem than you started with. Good luck...