if you want to. there are debates on whether 24/7 or 18/6 or 16/8 is the best. idk...i've tried the first two on my first grow and got dank buds. i really don't know that it matters that much other than ur electricity bill, how fast they grow (mayyyyybe), and temperature (depending on fixtures and positioning, bla bla bla).
anyway, 24 works as does 18 on and 6 off. i do the latter b/c i tell my g/f i don't grow any more.
lbb