I am attaching a new fex gas line to a new cook top. The new line comes with flare fittings that screw into the flex hose fitting. Should I use any sealant on the threads of the flare fitting. Thanks.

No. Teflon tape or sealant is not used on those types of fittings.

Rick's correct. Let me add that before you turn the gas on paint all the joints with liquid detergent. Now turn on the gas and check for any bubbles. If you see any you have a gas leak and you should shut down and repair it.
Always double check your work when working with gas. A gas explosion could ruin your whole day. Regards Tom