During my training I flew the PA28 Archer, with me + 1 large instructor and full tanks. This was considered perfectly normal. No-one checked the W&B (only 2-up, of course).
However, when I later made up a W&B spreadsheet I discovered that combination (he was a BIG guy) put us outside the balance limits, and we actually needed about 40kg of ballast in the luggage area to bring us within limits! Didn't stop the aircraft flying though.
On another occasion an instructor organised a day out in a PA28 with 3 qualified pilots, one leg each (one flying leg, not one-legged pilots, silly), as a confidence building exercise. We used full fuel on two of the legs. No-one did a W&B. I was surprised we used full fuel because I would have expected it to be over the weight limit. The instructor didn't seem to care about it (or more probably, had flown such a combination many times before and had no concerns about it). I later did a W&B, and yes, we were probably over weight (didn;t have everyone's weight to check properly). However, with adequately long runways (> 1000m) at each stop-over, there were no problems.
What suprised me, on both occasions, was the "disinterest" of the instructor to the W&B. Obviously they applied their "experience" to the situation.
GB.