DFC, en-route is not my area, if you pardon the pun. I'll leave that to others better qualified to explain how vortex spacing is applied then.
In an airfield approach situation in the UK the IFR minimum spacing is three miles. This means the vortex spacing for not only heavy aircraft, but medium* followed by a lighter aircraft or even a small followed by a light is greater than the IFR minima, therefore the vortex minima is applied.
* between two medium aircraft "When the leading aircraft is a B757, DC8, B707, IL62 or VC10, the minimum distance shall be increased to 4 miles"
Back to your point about the "heavy" suffix on initial contact being beneficial to you, if you join a frequency after the heavy has made initial contact, you won't then know it's category surely?