Chopin's Etude Op 25 No 9, (Butterfly) uses the 4th finger of the right hand when playing octaves in many places. This makes perfect sense when there is no pedal indicated.
But, in those parts where the pedal is used, is it acceptable to use the 5th finger?
If you can play this study beautifully, using your own fingering, any fingering, that should be good enough. But where it fits the hand, why not use 4? I'm looking at the Cortot edition (highly recommended; his fingering in particular is generally very very good), and in the very first bar the first pair of octaves is marked 5-5, the second 5-4. All of these are staccato, so there should be no problem hopping up from Cb to Cnat with 5-5, but coming down Eb-Db, then 5-4 seems more natural (to me). But this all depends on your hands.
FWIW, Cortot's introduction to this study says: "...demands a refined, humorous, somewhat carefree performance... (tone quality) precise and light". That's what you have to aim for. I really can't see that whether pedal is marked or not should affect the fingering.
Which edition are you using, by the way?
The main point of alternating fingers in octave passages is not to enable legato playing. It won't produce a good legato anyway, since the thumb always has to jump/slide (although for psycho-acoustic reaons, half a legato can produce a nice illusion of the real thing).
The real reason has to do with finger and hand positions. Judicious choice of the finger opposite the thumb can make a passage much easier to play overall, because the arm has to move less, and it's easier to avoid cramping if some finger muscles relax periodically rather than being active the entire time.
The fingering also articulates the musical phrase better. By using the fourth finger on some of the octaves, the thumb naturally accents the first note of each four note group. It also helps distinguish the third and fourth sixteenth notes in the groups.
