It depends, i know a lot of mix engineers and mastering engineers, more accomplished than you or i, who have things like...ya know...grammys for their work in music and music for major motions pictures who do not use crossfeed, or correction, while working on headphones
It really depends. With that said, id recommend giving canopener a try. I know more than a few mastering engineers working on headphones in their travel rigs who use it for crossfeed. I see you have an RME adi. Crossfeed software allows for a much greater adjustability in tailoring it to your ears than whats built it to the RME
It may be worth giving a look if you find crossfeed essential. Don't feel bad about your Phonitor 2 purchase. It holds it's value enough where you shouldn't have trouble selling it if it doesn't add anything to your mixing rig.
Below is a pic of my chain...soundsource for system wide, to sonarworks, into canopener. Sent screen shot of my settings in case you need a start point.
