You don't "need" to map it in if it's just for the sake of the sensor, all they do it remove the sensor reading from the program, it's cheaper to just buy a sensor spacer, if you having it mapped to benefit from the power, again not essential, you will see power gains just by fitting the exhaust, if it's been mapped already you'll gain a couple of ponies at best from the map, if it's not yet been mapped and you were wanting it mapped anyway then by all means go for it