The backward tilted accent is a "grave accent" and the forward tilting accent is called an "acute accent". It is actually considered part of the spelling of the words. Si with no accent means something different than Sì with a grave accent. Si with an accute accent (tilting forward) is just silly looking to italians. It's like in english mixing up there/their/they're. People still know what you're trying to say, but grammar nazi's will point it out and correct you. There are some where it actually makes a difference: chiése (verb, remote past of "to ask") ~ chièse (churches) pèsca (peach) e pésca (fishing, like the sport). They're also pronounced differently. Tonal differences.
si = himself, herself, themselves (no accent)
sì = yes (grave accent)
sí = not a word (acute accent)