Yes, your surrounds in a 5.1 set up should be positioned to the sides and slightly behind your seating position - leave a few feet behind the couch to the rear wall/open space - never put your seating up against a wall, as that gives poor sound.
The surrounds can be wall mounted higher if you like, angled down toward the viewers to clear some side space if you're able(?).
If wall mounting isn't an option, then consider some stands for the surrounds (and/or fronts), but try and keep the tweeters at about ear height, and all within about 18" of each other.
Ceiling mounts are another option, but are difficult to make work well enough sometimes...
Your front speakers don't have to be in corners, a foot or two from the corners
would likely yield better sound anyway.
'Toe' them in a bit toward the viewer.
Consider getting the book by Jim Smith called "Get Better Sound" (see his website), as there are MANY very inexpensive ideas on improving your system there, and its a great read too!