There is no right or wrong when it comes to isolators. Just like with HU and amps, not everyone uses the same stuff. Not everyone has the same subs, so not everyone has the same amps.
So, back to the OP, it really depends on your usage. Do you like to play with the engine off? Do you play until your battery is drained? If yes, then you should consider the relay (isolator). If you don't know what an isolator is or does, it is hard to know why you would need or want one.
Basically, think of any relay (isolator) as an automatic switch. When the engine is on, it is switched on. When the engine is off, the switch is off. The line that you are switching is the power wire from the front (alt) to the rear battery. When you turn that switch off, you are separating the two circuits. So, the front battery is no longer connected to the rear battery.
Why would this possibly be a bad thing? If you have only a small battery in the back and need power from the larger front battery to power your system, then an isolator would not work for you.
If you have a stock (flooded) battery up front and an AGM battery in the rear, you should consider the isolator because the flooded battery rests at 12.6v and the AGM should rest at 13.0v. The lower voltage battery will try to drain the higher voltage battery. This can happen in as little as a week without using the vehicle.