Here's a very good article from FIClinic.com on the value of using the correct injector offset when replacing injectors. And FIC provides these values specifically for the injectors you purchase which is a great service. So its not just a question of slapping the injectors in.
For those with larger displacement motors who are "running out" of injector,
using mixed sets of injectors is also not recommended since the offset values would be different for each set.
All of this has little effect on WOT, but significant impact on idle and part throttle, which constitutes the majority of engine usage.
http://fuelinjectorclinic.com/data-match-technology