Stefan, did you ever read the POH for the aircraft you're flying? It's a good habit to get into for any type or sort of aircraft you're flying now, or are going to fly in the future.
When you follow the advice in there, you really can't go wrong. Plus, the manufacturer has taken into account the type of engine (fixed or variable pitch, injection or not) and the instruments fitted (RPM, EGT, CHT or not) and tailored its advice particularly for that situation. So you don't have to read through two pages of how to lean for Peak EGT or ROP or LOP when you don't even have an EGT fitted.
And if anybody questions your leaning practices, you can simply point to the POH and tell them that you're flying the aircraft according to the manufacturers advice. That'll shut the majority of critics up.
I know that I still leave it on the rich side but without having been taught what to do I feel it's probably better to be on the safe side as I'd be worried of over doing it
A word of caution here. You need to either do as you're taught, or you need to follow the manufacturers advice - assuming those two are not the same. But if you're just going to be making up your own compromises you may actually be putting yourself in danger. This doesn't apply so much to leaning, although there may be a few corner cases where leaning "just a bit" may be more damaging to the engine than not leaning at all or leaning all the way. But it applies more to flying in general. If you make up your own compromises which are not taught or described anywhere, you are in fact a test pilot. (A far more extreme example would be an outside loop down. You either don't do it at all, or you need to do it with full gusto, headache be damned. But if you do it halfheartedly you'll find yourself exceeding Vne really quickly, with no easy way out. And the cows getting bigger awfully fast.)
Long story short, the bottom line is that the Mixture control is as much and engine control as the Throttle is.
Fully agree. Best performance for the lowest fuel burn requires careful management of MAP, RPM and Mixture. However, it also requires a fair portion of your attention, so you should also know when to compromise because your attention is required elsewhere. That's why, somewhere during the approach, we shove both RPM and mixture all the way forward. Two less things to worry about in case of a possible go-around.