Questions About 112 Mph

How important is it to match tire speed ratings?

As one who drives a boring 4 door sedan (non luxury, non sports) primarily to get to place A to B reliably and safely, I found that my car had a tire rating of "V" for speed (up to 149 mph). The auto store salesperson was adamant that for safety reasons, I should only buy a V, or at worst, one level down, a H at up to 130 mph at about a $30 premium over an S rated tire (up to 112 mph) with similar mileage warranty. Further he said I shouldn't even mix an S on one axle if I had two of the original Vs on the other axle (better tires on rear axle). Qn: Given that I never go over 75mph, and most of my drive is at actually 60 at best, how significant are these "race-car" or "immortal teenager" speed rated tires? Is it a slick way to pocket an extra 30-35 bucks from an easy mark such as me or is his statement true. If so, what exactly do these speed ratings mean? His second observation was little wrinkle- looking marks or cracks on the tire near the wheel. He said this was also a dang