Where possible I would say join as the operator reccomends or as directed by the AIP. However there will be situations where instructors or commercial pilots may need to operate in a more expidious way, so joining such as directly on to crosswind, For example a twenty minute trial lesson conducted by an to maximize the experience for the student to join on base might be reasonable.
The key thing in all of this is to maintain your own situational situational (SITAW)awareness by listening to the aircraft R/T calls and helping other pilots with their SITAW, even if it requires additional R/T call. This is based on there is A/G but no AFISO on duty.
The problem with instructors making non stadard joins, is that others you may considr themselves more exprienced PPL,s feeel they are above overhead joins. No the difficulty here is that if you join onto base or finals, the pilot is more likely to forget the prelanding checks. A case in point many years ago, at my local airfiled, one of the pilots, on returning from a business trips, he would regurlary call finals when I actual knew he was about 12 nm away, it was very disruptive to training and the instructors pointed this out to him in a kind way. Two weeks later I am on base leg, he calls long finals, I request his poistion, and he was 15 miles out, so I informed him since he was not even on a long final (4nm to 8nm), but still on a cross country, I had priority, and he would need to do a stadard overhead join as I was going to make a full stop land and 180 backtrack I though I had made my point.
A couple of months later same thing happens, I'm operating the A/G as an AFISO, pass the airfield information, I requests he calls long finals at between 4nm and 8nm, and finals when at less than 4 nm. I'm distraced for a few moments, and as I look out of the window, and I see the aircraft skidding down the runway on its belly. Apart from being a FI and an AFISO, was also fire crew, so we sped out to him, the triple bladed prop was bent, engine shock loaded, flaps ripped up and the fuselarge damaged.
The moral of this story is that if you do a non standard join you need to be sure yoour checks are completed, and the belt and braces approch is for the student or instructor on complex singles to make the call (not on R/T) as follows at around three hundred feet on finals.
'RED/BLUE/GREEN' (mixture, prop, 3 geens).
To add I have never agreed with the ATC/AFISO stating 'check three greens, unless the suspect the gear is still up.
Another accident I was aware of was a midair collision, one fatality and two survivors. A pilot in a high wing aircraft takes off from the duty runway and turns crosswind clinbing up to 1000ft, a high performace bi plane elects to join crosswind wind at 1000ft the same time, By the time they collide with each other, they are both about to turn downwind.
The AAIB cite the low sun as a factor, but also the somewhat late rejoin call from the bi plane. I would imagine that both aircraft in a time frame of 5 to ten minutes would have made 3 R/T calls each, yet neither were aware of the possible conflict. The AAIB pointed our that the late rejoin call by the bi plane was a factor. I do wonder whether a 'sterile cockpit' might have help avoid an accident.
So the sum up:
(a) Overhead joins are preferable for low hour pilots <500 hrs.
(b) Listen out to other aircraft, build a mental model of the other aircraft.
(c) Remember you can make additional calls such as base, late downwind, crosswind, if it is going to make your position clearer to other aircraft.
(d) Always have your landing lights on in the overhead and circuit, I know some clubs are against this on the basis of cost.
(e) Call up the airfield in a timely manner, I would say at least ten minutes, so you can build the big picture.
(f) Consider a 'sterile cockpit' in the overhead, rejoin and circuit, by that I mean only conversations that relate to the operation of the aircraft. This is what airline crew do below 10,000 ft.
(g) Give a wider berth for student pilots and thos with an Exam callsign.
(h) All the above relates to A/G airfiels where an AFISO may or not be there.