#Python 2, 88 bytes

Simple solution.

<!-- language-all: lang-python -->

	s=input()
	m=map(lambda x,y:y>x,s[:-1],s[1:])
	print all(x-y for x,y in zip(m[:-1],m[1:]))

[**Try it online**][1]

If same letters in a row was neither rise nor fall, the solution is 79 bytes:

	s=input()
	m=map(cmp,s[:-1],s[1:])
	print all(x-y for x,y in zip(m[:-1],m[1:]))


  [1]: https://repl.it/D7DP