This is a huge mistake because by the time the results of your "guess work" become obvious (should you have been incorrect) its already too late. Soil testing will not only tell you exactly how much N,P,K your lawns soil has in it but it also tells you where your soils pH level is. Too low or too high a pH level and your soils nutrients will simply remain "locked up" in the soil and be unavailable for use.

Grass will only absorb the nutrients that are available when the pH level is at the near exact level that your grass type requires it to be. Getting a handle on this issue at the very start of the grass growing season (before it enters into the semi-dormant period of July & August) will set the course of your lawn for the remainder of the year until winter finanly sets in.
The last thing that you want, is to enter the hot summer months with a weakened lawn which will already be facing the usual stress of drought like conditions.