I've been using typical solders breadboards and noticed because of a small voltage on my circuit return line that the buses measure ohms such as 15.6 ohms, 11, 17.5, 13.3 etc.. They might be older and with a little corrosion or certainly not gold plated. However my circuits on the ground line measure from a few millivolts to say 35 mv up to 90-130mv depending on the circuit. Yes, the voltages diminish when I build a soldered version of my circuits. I would like to ground my scope's probe closely near a particular chip's return to ground for a more accurate measure but since since looking more into the small bus return voltages have been hesitant to. I have been using a jumper to near the power supply's return.
Theory tells me that my power supply and function generator are floating so grounding the scope's probe such that there is would be a small measurable voltage with reference to the supply's return wouldn't harm the scope at all. After all it is the only earth ground. But I'm a little hesitant to and thought I would ask some others to weigh in. I doubt that I'm the only one with these "large" ohm rises with solders breadboards.

 

Ground as close to your circuit as you can. Plug in type breadboards are notorious about this problem. Once you have your circuit working as well as you can with the breadboard, jump to a perfboard and solder construction to finalize your design.

 

Thanks, so I do take it that the scope's grounding at earth level at 10mv to 100mv above the power supply's negative voltage is fine since the power supply is floating.