I've always transferred 2 whether they be day 3 or day 5 embryos. Out of 10 cycles I had a singleton on cycle 1 (2×day 3), a good few miscarriages, 2 bfn's and cycle 10 produced originally a twin pregnancy but lost 1 at 5+3 weeks and i ended up with another singleton. Those were 2× day 5 embryos.
I think its down to the individual and their own personal viewpoint. For me, 2 was always the way.