I actually disagree slightly with @Joe Strazzere here, on the 'anger' part. I believe the legal part is correct, that most locations there is nothing binding until / unless you sign. Also, I don't believe many places would actually enforce even a written offer acceptance if no money has changed hands (most locations in the US actually require money to change hands for a contract to be enforceable). My company does not count someone as 'hired' until they are through Day 1 inprocessing, and we don't count them as 'offer accepted' until paperwork is signed. Further, many companies don't even do the background check until after you're hired, but have the offer stipulate something like "subject to background investigation"
As a hiring manager, I'll tell you that the degree of irritation over any declined offer (whether before or after the acceptance) is directly proportional to the fill-ability of that position and the number of irons I have in the fire.
If I'm doing a 'massive hiring' for a 'floor level position', and one guy backs out because he got a better offer before he started working for me, I might have a brief moment of 'what the hell, dude?' before moving on to the next candidate. But no anger, and certainly no grudges. I probably wouldn't even remember you six months later (not that you're not memorable, my friend, just that we're talking about a situation for a standard floor job, which implies low barriers to entry. In those situations in the past, I've had 10-15 applicants for every offer acceptance, possibly more. ) "There are more fish in the sea" as it were.
I'll tell you that conversely, I would feel pretty bad about someone taking my entry level job if they were qualified for and had been offered a more advanced position elsewhere, and I found out about it. Hell, I've had folks tell me six months or less after being hired, "Hey, I am really overqualified for this, and I have job X offered to me" and I wish them luck, exchange contact info in case they change their mind and try and keep in touch with them on LinkedIn, etc. There's no point holding grudges, and it's a lot easier to build animosity doing something you're not satisfied with.