Geoff,
In my experience there aren't all that many occasions where the 5th row is absolutely necessary, unless you have learned or taught yourself to change rows according to which key you are playing in. I do believe you have to play along with other instruments, which is something I've never done with anything other than a guitar, and we all know we can alter the tuning of those with relative ease.
In most English speaking countries there were very few teachers who would entertain anything other than 5 rows, but in the countries where the CBA is the most common type of accordion, there has always been a tendency to teach pupils to get everything down on three rows only. A lot of Russian bayans only have three rows, and you are bound to have seen photos of Yvette Horner and other French pros taking the stage with 3 row boxes.
There is absolutely no doubt at all that players who can play their whole repertoire on three rows only achieve better flexibility and strength in all 5 fingers of their right hands, compared to those of us who love to go a wandering all over the buttons. Quite a few players end up with little or no strength and flexibility in their little fingers if they use all 5 rows, and whilst I am in no position to be critical of the hundreds of excellent players I have seen using the 5 row technique, it doesn't quite cut it if you want to play the old style French musette repertoire. Even the very few old school French players who made prolific use of their thumbs seldom used the 5th row, if they had one at all.
I started off with five rows, but after I was able to watch a lot of players on videos, and later You Tube, I realised my technique was not quite right for the style of music I was attempting to play. Eventually I disciplined myself to come off the 5th row altogether, and only use the 4th row if I found I was ending up with my fingers in an awkward position.
If you are up to persevering you might want to try and learn those tunes in three different positions, so that any change of key will come naturally in time. Having said all that, my main instrument is actually a 5 row, but all the 5th row buttons do is make the box look more complicated than it actually is.