Balance transfers won’t have any negative effect on your credit score as long as they’re approved. While you may get hit with an additional hard inquiry on your credit report, the new credit line will improve your credit utilization ratio, which is more heavily weighted on your score than inquiries, so it will cancel out any hit taken because of the inquiry.
However, if you’re denied a balance transfer, you should expect to see a small dip in your credit score because of the new hard inquiry. This only accounts for 10% of your total score, though, so don’t worry about it causing too much of a decline.