When having windows installed, is it foolish to do have them do it without the flange? They are offereing lifetime warranty on the installation.

I am having new windows installed and I can choose whether they install with the flange or without. It costs more to do it with and then there is the extra trim work I will have to do as well. Or, they can do it without the flange. Either way they offer a lifetime warranty on installation. To flange or not to flange. That is the question.