If you're positive that you will never use this code in a compiler that doesn't support it (Windows/VS, GCC, and Clang are examples of compilers that do support it), then you can certainly use #pragma once without worries.
You can also just use both (see example below), so that you get portability and compilation speedup on compatible systems
#pragma once
#ifndef _HEADER_H_
#define _HEADER_H_
...
#endif
