Components on the - or + side?

does it really matter? I mean i can still get the same effect if i plug a resistor on either end , because im reading a book on the Arduino and all the examples the illustrations show components connected to the ground , and im not sure where i read something about current vs actual current , the current flows from negative to positive but that somewhere else its always depicted the other way im not sure what i read, but at low levels of electronic engineering , and by low level i mean i will probably never go over anything higher then 12v , does it really matter where my components go?