I don't know whether or not they are genuine but I don't think that's proof.
If it is a phrase he uses (frequently or infrequently) then he could easily have his secretary write it.
Just like Colin Firth's does with his secretary inscribing with "best wishes".

I know some people are unsure if Josh Dallas sends genuine autographs or secs, but I just saw a tweet he sent to Kelly Clarkson which reads:

“@kelly_clarkson: @joshdallas congrats to you!!” Thank you! And a huge congrats to you!! We'll have to compare notes! Big Love J #amazing

Now, the autographs are pretty much always inscribed "Big Love" and he has used the phrase "Big love" in that tweet. I already believed he signed genuinely. but that is further proof. It's not like it's a catchphrase he uses all the time, so I don't see why a secretary would feel the need to include it.