How prominent are surround speakers supposed to be?

[Cisco in HD]

First I hope this in the right forum since I'm speaking of volume rather than a specific brand.
Now I know how to tune speakers db wise with an SPL meter, now I want to know from people a bit better versed than I is, how faint/prominent are surrounds supposed to be? I understand that surrounds aren't supposed to "noticed". Then again it all depends on personal taste. The 85% or more of the sound stage comes from the front 3 and the back speakers are for "support". The reason I ask is because I watched part 6 of BoB: Bastogne today and the bullets whizzing by and the plane fly overs sounded a bit faint. Is there a general rule about how loud the rears should be? Is there supposed to be a db difference with the front and the rears? If so what's a good difference, for example.