When I worked in the UK (a long time ago now) I used to try and hit the CTOTs on the money but there are times when you have two or three departures with the same CTOT going to different places and some unrestricted traffic and you used the window accordingly.
I thought then, and still do now, that a 15 minute window (with the +15 some airports were allowed to utilize) on a flight as short as an hour or so, was hardly a guaranteed method of smoothing the flow down the line.
Where I am now most flow control is dynamic. Times are requested on taxy and tolerances are much finer. +/-3 minutes for most destinations and ORD is just -3. A 3 minute slot window should provide a better flow but it does mean that on a mixed mode runway you have at best two gaps to hit it and sometimes just one.
When Ground Delay Programs (GDPs) are in effect we are expected to hit the time as close as possible, all things considered.