As a general rule, even at low voltages you want to be careful where you put the grounding clip of the scope probes. Not to protect you, you'll be fine, but to protect the scope. If the thing you are measuring, low voltage as it might be, is relative to mains earth, and not floating, then putting the grounding clip to somewhere on it which isn't at mains earth makes a current flow through the grounding clip, as a dead short to mains earth. This can hurt the scope if the current is large, not sure if there is a figure for what scopes will survive, but whether the thing supplying the current is current limited will be important. Most wall wart supplied power is floating, when power for a circuit comes from a computer's USB port it may or may not have a floating ground, depends on the whether the PC is a desktop (USB ground usually connected to mains earth) or a desktop (whether the USB ground touches mains earth depends on exact model of power brick), bench supplies will always have clearly labelled options for connecting up both a floating ground, to which the DC supply pins are relative, and a mains earth ground. Anything solely battery powered is always floating. When you're new to o-scopes, use only one grounding clip, this prevents the risk of accidentally connecting each clip to different voltages, which even on floating low votage circuits could be bad. You only need both grounding clips when you're measuring fast signals (100s of KHz and above) and need to have reduced capacictance to avoid parasitic ringing in your measurements. Except when you're working on things related to mains, most circuits you are likely to want to build will be floating, so as long as the power supply to them is floating you need not worry about mains earth vs circuit ground, because for a floating circuit, wherever you choose to connect the scope grounding clips to will instantly and harmlessly become mains earth, and all other voltages on the circuit will be relative to that.