A stiff, heavy trigger pull could very possibly have an adverse effect on your shooting.

I wouldn't touch the trigger on a Ruger M77 MkII or on my Remington 700s, but I had no difficulty whatsoever adjusting the trigger on my Model 70. I think the Model 70 trigger is the best factory trigger out there. Mine came at an awful 7+ pounds from the factory, but I adjusted it myself to about 3.5 pounds. If you follow the instructions that Skeeter's link gives, you should not have trouble.

I would recommend using a trigger pull guage. You can get one of these for less than the price of a trigger job.

Again, if you are not mechanically inclined, or are the least bit uncertain, let the gunsmith do it for you. If you are comfortable taking your fishing reel apart to clean it, you should not have any trouble with a Winchester trigger.

Good luck!