We design all of our products on fully stock bikes (bikes that have not yet been tuned). For some models, it is best to tune to get the most performance out of the new setup, but it is not necessary. This is mainly for full systems.
For slip-ons, since you are only replacing the stock mid-pipe and muffler, you do not really need to re-tune. If you are looking for a little more performance, then you could think about adding it (but it is not necessary).