I too have always been told that when you need the toilet, you should go. This was, of course, not always permitted in my school-days, but they are long since over.
Either going as soon as you feel the need is healthier, as I have not yet been subject either to infections of the bladder or to constipation, or I have just been lucky.
I think the older we get, the oftener we need to wee, so having been uncomfortable more than once when in places were there were neither public toilets, nor convenient bushes, I bought a she-wee, which I take with me when going out shopping, etc. as using it a solitary tree can provide enough shelter for neither my nor others' modesty to be offended.