Best practice to switch +V or GND connection?
In practice it is best to switch the hot terminal, +V in your case. Ground is often connected to safety ground and as such has same potential as the world around you. When you unscrew a light bulb from its fitting, you don't want to accidentally get zapped when you touch the metals.

BTW it it also good practice to connect null to the outer terminal of the light bulb because of exactly the same reason: it is easier to accidentally touch.

With electronic circuits on the other hand, often the low side is switched with a transistor. This is in part because old logic parts were much easier made to be able to sink more current to GND than they could source current from Vcc. This is in turn caused by the fact that it used to be easier to manufacture NPN transistors and N-channel MOSFETs. Today PNP and P-channel MOSFETs are not too much of a problem, but you can still find better specs for NPN and N-channel in the datasheet. The main difference is with the 'majority carriers', either free electrons (for NPN and N-channel FETs) or free 'holes' (missing electrons for PNP and P-channel FETs). Electrons move quicker through the semiconductor material than holes. Hence low side switching with transistors is usually preferred.