Quick question on the tonal fugue

Ok, let's say I have written a plain jane no accidentals subject, all in key notes. I give a tonal answer transposed up a fifth to the dominant region, changing notes to make it look more like it's implying the tonic chord. here's the catch, as I go about doing this, and go about the fugue in general (such as playing the subject in the subdominant).....in a tonal fugue, are you supposed to transpose not only to the region of the subdominant, but the actual key, to preserve the intervals, or do you keep it all in the home key?

let's say I'm in C maj, and I have a 5 note subject, C E G F B (heh). When I get to the subdominant, would it just be F A C B E, or should I actually rewrite it in F maj to preserve the intervals? (F A C Bb E). I've been looking through Bach fugues to see if he does one or the other, it seems he throughout any given fugue, 3/4 of the time he transposed into the actual key, even when just going to the dominant (raising F to F# if playing in G over a fugue in C). I thought that was a chromatic/real fugue, not tonal? I could understand going into some chromatic alterations/other keys during episodes and whatnot, but how can it really be a tonal fugue if you alter the subject literally every single time it appears in the work? take this one for instance:

http://imslp.info/files/imglnks/usimg/c/cd/IMSLP01316-BWV0531.pdf

You call an answer a tonal answer if the subject is changed so that is sounds in the initial key. (C E G F B might become G B D C F)

A real answer has the notes transposed exactly. (C E G F B becomes G B D C F#)

so the fugue I linked to should without a doubt be considered a real fugue then? It flats the B, preserving the major third of the corresponding phrase (A-F becomes D-Bb in the answer) so that would be in the subdominant F major, rather than just a dominant-tonic answer?

Yeah, I guess so.

Er, I can't find an outside source on the internet that says it has to be in the dominant. Only "usually on the dominant". Could you show me?

I read up and I think John is right. But thanks for making us research things readily available to you.