Should you turn off the main water supply if your pipes freeze?

Think about it this way... Imagine you try to take the sprayer off your garden hose that you used to water the lawn and want to replace that with a nozzle to wash your car. What happens if you do not turn off the flow of the water? The pressure is so strong behind the nozzle that it makes it hard to install, and you are likely going to have a lot of extra water spraying the lawn and your clothes before you manage to get the nozzle on.

If the pipes are frozen now, the water supply coming into your home is likely blocked and waiting for the moment that it will be free. It is under a certain amount of pressure, but not enough to push the ice out of the pipes. Now if the pipes thaw out a bit, enough to let the water start flowing again, and if you do indeed now have a pipe that has ruptured, when the thawed water reaches the spot of the ruptured pipe, you will suddenly have anywhere from a drip to an arcing spray of water all over that area of your home.

If no pipes have burst yet, turning off the main valve just eases the pressure BEHIND the frozen section. But generally, no it does not help prevent the pipe from bursting. Turning it off is mainly so that IF you do indeed have a burst pipe, you won't suddenly have the water from the municipality filling up your basement or crawlspace.

Generally accepted for resolving a frozen pipe is to shut off the main valve, open the faucet or valve for the frozen section, heat up the section that is frozen, doing so as evenly as possible (so no blowtorch on one spot for a long time) and then when you see water is dripping or running out of the open faucet, try turning the main valve back on and look for leaks.

Do that if you either know a pipe has already burst, or can't tell yet, or won't be there when it thaws. Then it's your hedge against flooding. Main thing is to ramp up the heat down there, wherever "there" may be. Go after any pipe going through or against an outside wall from the warm area of the home. Top picks for me would be an unprotected (non-freeze proof) outside faucet supply line, unheated crawl space lines, and under the sink supplies in a closed cabinet against an outside wall. Magic Q: How do you know they are frozen?...

it will prevent leaks that will happen when the freeze melts; you have a lot of pipe to replace before then

Turning off the water supply will not prevent it from bursting as it probably has already burst. However it will prevent the frozen pipes from flooding your crawlspace once the pipes thaw out.

yes