The other day I took my car in for an oil change. While talking to mechanic about tunning my car, he suggested me to change the spark plugs. My friend who is heavy vehical mechanic told me that I need not to change the spark plugs if there is no problem with starting the car. I drive 1998 Saturn SL2 with 155k on it.

Would changing the spark plugs change the mileage of the car?

If you don't know when they have last been changed, it can't hurt. It takes as much time to change plugs as to check plugs.
Worse thing can happen is you will have a spare set of good plugs?

The mechanics suggestion for the tune-up was in the form of preventative maintenance.
If you had a misfire or skip, then would be the time to have the secondary ignition (plugs and wires) checked.
And a simple tune-up like this won't have any appreciable impact on mileage. A clean fuel system and a light foot, both throttle and brake, will improve your fuel mileage.
And those driving habits could also lead to improved insurance rates, if you know what I mean.

Thanks folks