The 27/34 is probably for carrying heavy loads, but I would recommend about 32 all around for normal driving and light loads. Higher pressure reduces rolling resistance for better fuel economy and lower tire wear and crisps up the steering response. Higher pressure also increases the safety margin, because the tire runs cooler and there is less chance of a structural failure.
Most OEMs recommend minimum pressures for ride comfort. That's what Ford did with the Explorer with non-speed rated tires. Then people loaded them to the gills and drove in summer weather at 85 MPH. Notwithstanding the manufacturing defects, which were an issue, the basic tire design and pressure recommendations were marginal at best, especially when most car owners can't even remember the last time the tire pressure was checked.
Also, on the S-class a 205/65R-15 does not have that high a load capacity tire considering the curb weight and gross weight when you load it up. For a given tire size load capacity is a function of pressure and the maximum load capacity and maximum cold pressure is molded in the tire. I like to have total max load capacity at least 50 percent higher than curb weight, but some OEM sizes don't make this cut.