How important is it? Well, that's really something only you yourself can answer, as it differs for each person. I, for example, come from a family where though we have a fantastic relationship it is almost unheard of for any of us to say "I love you" in a non-romantic situation. We mainly show it through action, and frankly for me the thought of saying "I love you" to anyone at all is both foreign and awkward. Something that makes me doubly wary of the phrase is the fact that when I was in my late teens, I used it far too liberally in a relationship and really hurt a woman I cared about. Ever since them I've been extremely wary of the phrase, not using it (same with the phrase "in love", for that matter) until I'm completely certain of it.
In keeping with the mistake I made back then, I will caution you on the fact that words are in fact cheap. Especially amongst the younger crowd, like teens and early twenties. As people age, the people who use the phrase without regard tend to be more blatant in their foolishness or self-interest.
In short, the answer is something only you can give. Keep in mind actions speak louder than words. If actions alone are not enough, however, talk about it with your significant other. You might be amazed how many things can be resolved by simply putting aside shame and pride, and just sitting down and putting the cards on the table.