It depends on what you doing, i normally put mine under the engine cross member.
You have to be VERY carefull putting them under the chassis rails as you could run the risk of them slipping and going through the floor.
What I'd say is if useing the chassis rails as a jacking point, see if you can find a double layered point near the front of the car as this will be stronger, but be very carefull and use some wooden blocks also i use a trolley jack under the engine cross member as an additional support.
If you have any doubts about useing chassis rails to jack or support a car don't do it is my advice.