Grumpy_Mike:
I know people say its due to resistance, but by what formula?
Ohms law, suppose you have a ground lead with a 1A current flowing through it an a 1 ohm resistance.
Then at one end of the ground lead, the one at the supply, will be 0V and the other end will be 1V. Half way along that lead it will be 0.5V. So if you connect something else half way along that ground lead it will not be ground but 0.5V.Therefore connect all current carrying ground leads together at one point. This is called a star configuration.
Aha! The number of different articles I've read - but how that was worded for some reason makes perfect sense to me. It also explains perfectly why a number of devices I've worked with have two ground connections on the board back to the power supply, one for the high current devices which won't be 0V ground, but lifted, whilst the other ground for control devices will be 0V (well, much closer!). I assume that's exactly what you're referring to when you say "therefore, connect all current carrying grounds together at one point"...
You're not so grumpy after all! 