The idea in getting you to fly 160 to 4 is that we get everyone to fly 160 to 4 so that we can keep the spacing constant so we can land more aeroplanes, so airlines don't moan at us.
It matters not to us what speed it is, be it 170 to 5, 155 to 3 or whatever, but the more people who fly the same speed the better.
Saying that, though, as has been mentioned above, if you'd rather fly something other than 160, let Approach know ASAP, and they'll fit you in.