Is there a danger of back flow from water heater while replacing a saddle valve?
Yes, it's good possibility that if you don't purposefully depressurize the lines first, you will get spray. But this is easy to do.


Turn off the hot water heater. If it's electric, there is usually an off position on the knob on the front, or turn it off at the breaker panel. If it's gas, just turn the knob to pilot.
Turn off the cold water supply to the house.
Open the hot and cold taps in a sink somewhere. When the water stops flowing, you're good. If you want to be really safe, you could drain a little bit of water out of the tank using the valve at the bottom.
Complete your repair to the saddle valve.
Turn on the cold water supply to the house.
Open the cold tap on a sink. Wait for air to bleed out and a good water flow. Repeat with the hot tap. The hot water could take a much longer time to flow properly.
Turn your water heater back on.