Why does the T7 RNA Polymerase require a reducing environment ie. DTT
A quick search on T7 cysteines gave some clues:


  Bacteriophage T7-induced DNA polymerase is composed of a 1: 1
  complex of phage-induced gene 5 protein  and Escherichia coli
  thioredoxin. Preparation of active  subunits in the absence of 
  sulfhydryl  reagents  indicates  the reduced form of thioredoxin  is 
  sufficient for formation of  the  active holoenzyme. The oxidized
  form of  thioredoxin, thioredoxin modified  at  one  active  site 
  sulfhydryl by iodoacetate or methyl iodide, or thioredoxin modified
  at both  active  site  sulfhydryls  by N-ethylmaleimide,  are  all 
  inactive,  being  defective  in  complex formation with  gene 5
  protein.


Adler and Modrich, J Biol Chem 258:6956 (1983)

There's a more recent paper (Aguirre et al, Inorganic Chemistry 48:4425 (2009)) that mentions the "the enzyme critical sulfhydryl cysteine groups", but unfortunately I only have access to the abstract.

Update: It seems to be an old finding, rather than a rationale concerning the cytoplasmic redox state. According to Chamberlin and Ring, JBC 248:2235 (1973), 


  General Requirements-The general requirements for T7 RNA 
  synthesis directed by T7 DNA polymerase are shown in Table 
  I. As expected for a template directed polymerase, RNA 
  synthesis shows an absolute requirement for DNA, the 4 
  ribonucleoside triphosphates and Mg++. 


(no surprises there ;)


  The activity of the 
  enzyme is reduced significantly if a sulfhydryl reducing 
  agent such as b-mercapto-ethanol is omitted from  the 
  reaction. The addition of 10^-5 M p-hydroxymercuribenzoate to
  the assay system in the absence of b-mercaptoethanol 
  abolished all activity, indicating that the enzyme contains a 
  sulfhydryl group necessary for activity.


However, if you see the table I, the remaining activity after removing bme is still 74%

There seems to be 7 exposed cysteines (Mukherjee et al, Cell 110:81 (2002)), but I could not find any paper discussing their roles.