The Apple docs on receipt validation say to perform receipt validation immediately after launch. This amounts to checking for data at the path returned by
[[NSBundle mainBundle] appStoreRecieptURL]
you don't need to do so, that is optional only and could be done on iOS7+, if you interested in doing that.
briefly, implementing the validation is purely a financial decision, and even if you validate recipes you are recommended not to disable the content in case of failure as the validation could fail in standard environment as well anytime (e.g. in case of no connection), and such overreaction may ruin your consumers' experience.
altogether, doing the validation'd rather make sense on OSX in practice when you are allowed to disable the content in case of failure, regardless the reason; but if you feel you have more consumers than your income suggests or the amount of the stolen content is way beyond your margin, it may be worth to do it on iOS as well.
NOTE: in general you can read more about the technical details of receipt validation in Apple's Documentation.