This is going to be a personal preference. The room my rack is in isn't kept warm and averages about 23°C so I have heat mats that cover half the bottom and goes up the sides of the tub aswell to keep the heat in the tub. This works perfectly for me but not for everyone so again it's going to be personal preference and will differ on everyone's situation.
When I first started I was told the rule of thumb in one third of the bottom of the tub to be covered with a heat mat which is great if your room is kept at a perfect temperature. Which mine is not lol