1. Current balance transfer and check writing deals with many CC companies is 100% interest and fee free if you understand the terms and use accordingly. So, tax deductibility or not, if you pay with any type of interest bearing instrument, the 0% offers are better. I have about 3 floating right now with 0% interest for 6 months to 1 year. I've actually taken free money from one, then applied for a new card when the time was running out, and transferred the remaining balance. Citibank was having a 3.9% rate with no time limit - you just had to pay the minimum 2% a month and then you also had to not keep any other balance on the card - otherwise that was financed at 10% (that scam thing - but you could work around it if you wanted to).
2. Folks looking to pay with AMEX, Discover, or other CC not using an interest free option are generally going to pay it off immediately and only using the CC to get some other benefit - with AMEX and some others you get frequent flier miles, Discover gets cash back, others use their Upromise or BabyMint. If you have the money to pay it off, it makes sense to charge to one of these CC and then pay off the CC company taking their benefits for simply letting the money go through their hands (taking their merchant fees from the company along the way). Likewise, if you are doing the balance transfer deal, instead of paying DVC directly with a transfer check, it is better to charge to one of the cards with an extra benefit, and then transfer that balance.