Many, if not most, golf courses are measured contrary USGA recommendations.
ONe frequently sees courses that are measured from the back fringe of the teeing grounds. This practice could be described as vanity on the part of the course, making it appear longer than it actually is, since noone ever playes from that exact spot.
Matt Cohn mentions above a course where the markersa re at the front of each teeing ground. Interesting.
The USGA suggests that a course be measured from the middle of each teeing ground. This practice creates two outcomes: 1) if all courses were measured this way, slope ratings would be more consistent; 2) the tees could be moved somewhat forward or back without egregeously affecting the effective distance of the hole. For example, I have played courses where the yardage marker was at the very back of the tee, and the tee markers were twenty yards ahead of it. Had the yardage marker been in the middle of the tee, the tee markers could be placed ten yards either side of it and the relevant distances (carry, to obstacles, etc.) could be easily converted. This is the procedure, BTW, that the USGA and other savvy tournament organizations use to set up courses for competition.
To the point, however; I have always considered the "tips" to be as far back as one could play and have a teeing ground which is on the area prepared as a "tee" (i.e. not standing with one's back foot ofr the constructed tee or in any rough) and conforms to the rules of golf; that is, an area not less that two club lengths in depth when measured from the forward edge of the tee markers.