Whether to connect your signal ground (digital ground) to earth or not is not only a question of individual taste, but also a safety issue: If your power supply is 'safety class II', provided by double or enhanced isolation, then you need not to connect signal ground to earth (green-yellow wire of mains). But if your power supply is 'saftey class I', then you must connect it to earth!

The idea behind is: What happens if the basic isolation between mains and your digital supply voltage fails, due to a heavy overvoltage spike on mains, an unnatural overheating of an important isolation measure, or something similar? If there's still another isolation barrier, due to the use of a double or enhanced isolation, because your power supply is 'safety class II', then there's no significant danger of getting any risky potential on your (digital) power supply. But in case of a 'safety class I' supply, where only a more or less simple basic isolation might exist, there is such a significant danger! Only remedy then is to connect signal ground to earth.

By the way, sometimes it can be helpful to have the option of connecting signal ground to earth even when using 'safety class II' power supply in combination with isolating plastic enclosure. In this case green-yellow wire has no safety purpose, and because of this it's no longer called 'safety earth', but 'functional earth'.

It's often the mains transformer, which provides isolation between mains voltage and circuitry connected to secondary winding. How to see, whether it's 'safety class I' or 'II'?
It's stamped on the transformer.
Standardly used miniature transformers encapsulated in isolating potting material are mostly 'safety class II' transformers. Here, enhanced isolation is provided by having the primary and secondary winding separated by a certain safety distance. Each winding is mounted on an individual and isolated frame. Additionally, the whole transformer is put into highly isolating potting material. Such a transformer is often called 'safety transformer'.

Toroid transformers cannot be isolated such easily. The whole isolation can only be provided by one or more isolation foil(s) between primary and secondary winding, which are wound on each other. So, for many years only 'saftey class I' toroid transformers were available. But since some years even 'safety class II' toroid transformers are produced. They make designing sound and stage applications a pleasure...



Having something really floating isn't a pleasure either. Think about all the capacitive stray coupling which ends on the floating ground and cannot be shunted to earth. Think about ESD events hitting your floating ground.

To the potential on mains earth:
I have never measured more than 1V in home applications, and never more than 5V in industry applications. But, there's no guarantee for any limit! And there's no limit for the currents which can flow due to ground loops either! So, to have signal ground connected to earth must be discussed very thoroughly from case to case. The advantage is to have all benefits of a continuous Faraday cage, being able to shield even against cell phone radiation, which is always a mess with well known 'soft earthing' measures, like the use of 10...100nF capacitors and such stuff.
The disadvantage is the danger of eventually destroying ground loops...

Kai