Personally, I'd think the forward-pointing array at the navigational deflector location would be ideal for very long range scanning: the instruments would get a boost from the FTL fields that the deflector projects ahead of the ship. Such scanning would be useful for deep space exploration starships such as the various Enterprises, but less necessary for survey vessels such as the Oberths that first putter to the survey location and settle in orbit, then spend weeks or months doing short range scans.
It may be incidental or coincidental, but ships mostly seen in combat roles (say, Akira or Defiant) seem to have somewhat smaller glowing thingamabobs than their exploration-oriented contemporaries and stablemates. And the E-E, which seems more martial than the E-D, also goes for a more compact dish...
That's very interesting, Timo, about FTL-field assisted scans.
Would also the crusiers (larger ships with secondary hulls, bigger, wider spread engines and struts), present a bigger surface area to 'deflect' objects from? There may be multiple ways of clearing debris from space, but there's probably a 'best way' when a ship presents that big of a cross section as the Enterprise or other ships with secondary hulls. 'Big' is relative of course, to the era.