Using a balance transfer card when there is no fee on the line is one thing, but be warned, balance transfer cards that tout an intro offer of 0% interest and $0 fees won’t last forever. After the intro period expires, APRs can (and often do) skyrocket. Furthermore, if you have the rare card that doesn't charge a balance transfer fee for a period, be aware that period is usually only 45 or 60 days from account opening. After that (and with the vast majority of balance transfer cards from the beginning) you'll pay a 3-5% fee for each transfer. In other words, use your card responsibly and understand what the cost of your balance transfer transaction will be each time you intend to do one.
On the other hand, it's not uncommon for a balance transfer card to be a rewards card, too. While you won't earn rewards on the amount you transfer, on an ongoing basis you will earn rewards on any other purchases you put on the card, which is a nice incentive to use the card long term.