Possibly, but I suspect she was chosen because she's an excellent character actress, not because her birthday coincidentally falls on the same day as Damien's.
I'm surprised that this didn't come up in the programme on TV the other night called "The Curse of the Omen", which listed all the coincidences and various accidents that were associated with the making of the first Omen film.
I guess you can read significance into any numbers,but add the digits Billie Whitelaw's birth year 1+9+3+2 = 15, add again 1+ 5 = 6.You now have a 3rd six!!