I am a huge fan of Pauls and been quite lucky to visit his studio, as we have seen in his videos, he uses a straight holder for Copperplate and an oblique holder for Spencerian- if I am not mistaken he does this, so that he is prepared for the job in hand). With his work load etc plus the cost of Vellum, you cannot afford to make any mistakes. He is not the only calligrapher who is an advocate of a straight holder for Copperplate- please take a look at the work of Rachael Yallop, whose work is fantastic, plus Hamid Reza Ebrahimi also uses a staight holder for his work- to name but a few.
Having said that, when I started out last year- and living in the UK oblique holders are not that easy to be found, especially if you do not want to pay the duty or shipping cost.
To sum up, I believe that you should use the tool that you are most comfortable with- the tool that will get the job done- whether that be a straight holder or an oblique (or a quill)
Saron