I float money on these zero interest cards regularly (usually for house renovation expenses). It's cheaper than a HELOC although a bit more stressful. I keep a few things in mind:
Keep your overall credit utilization low. I typically will use a new card until the balance is perhaps 25% of available credit on that line, then stop and let it ride through the zero interest period, paying only minimums. I return my normal spending to a bonus card (most of the zero percent cards don't offer great bonuses) and pay it off monthly.
If I want to "transfer" this zero interest balance, instead of doing an actual transfer and paying a transfer fee, about 3-4 months before the zero interest period ends I apply for another zero interest card. Start using this one the same way, build up a cash surplus, and pay off the first card just before the zero interest period expires. Repeat.
This requires some organization, although if you're already churning the rewards cards, you're probably already keeping track of this stuff.
