Toilet porcelain is thick, but it is not designed to withstand sudden, extreme temperature changes. Pouring boiling water into a cool toilet bowl can cause the porcelain to crack or shatter. A cracked toilet bowl is a far more expensive and messy problem than a simple clog.
The hot water trick is a fantastic, chemical-free way to handle soft toilet clogs, provided you follow the safety rules. Remember: pour hot water down toilet