PaulS:
Will I damage anything if I connect Uno to PC with USB cable (that also has 5V supply)?
No. The Arduino has a power selector circuit that handles two separate power inputs. When USB power is present, that is used. When it isn't the Vin pin power is used.
Actually external power has first switching priority over USB power. USB power is used if only there is no external power avalible.
However it brings another layer of complexity to the game. He is powering his shield via the Vin pin, that then activates both the on-board +5vdc on the Uno board and the +5vdc regulator that he has installed on his shield board. So while the USB +5vdc is isolated via the auto-voltage switching circuit on the Uno board, he is still 'hardwiring" the outputs of two different +5vdc regulators together. And I'm sure that I"ve read in regulator datasheets in the past that they do not recommend that practice. However it will probably work fine even if not a good engineering practice.
One way to eliminate this possible problem is to make sure that the +5vdc power on your shield board does not connect to the +5vdc shield connector power pin. That would keep the two regulators isolated from each other. But then he would have the problem of his shield would not have any +5vdc power if he was running on USB power. Oh the evil webs we weave.
Let us know if it lets any of the magic smoke out. 
Lefty