Does anyone recommend using joint stereo? I've heard good and bad reports on this. Does it affect the sound (as much as you would notice it) when you play it on a normal hi-fi?

Well, it depends on the bandwidth. If the size of the encoded files does not matter, you can encode at 256 or 320 kbps. In this case I'd recommend using two-channel stereo. You may want to test between 192 and 256 kbps. Everything that is lower takes joint stereo IMHO.

Check the above link

I know this. But I seem to hear the differences between true stereo and js afa the positions of the instruments is concerned. They appear much clearer and better defined. But this is purely personal.

This has been discussed many times here.

I learnt to play the piano some ...ty years ago, and I still practise the guitar every now and then. This might influence my opinion.

Joint Stereo will play fine on Hi-Fi.

Is JS recommended?

Certainly, if you use LAME with the Presets.

Be more careful when you use Joint Stereo with Fraunhofer-encoding processes. Fraunhofer has been reported to be somewhat troublesome with JS. This encoder may have overcomed the problem, but who knows?
So, besides LAME + Presets, for other encoders, if you're not sure, just use Ordinary Stereo.

joint stereo is the HIGHER quality option if you use LAME (which you should be using anyway if you're concerned with quality). the JS implementation in LAME maintains stereo separation while still using bandwidth more efficiently. here's a list of recommended LAME settings.