Any form of rebellion in the society of 1984 is definitely significant, because it is putting your own life and sanity at risk. In their society, people are so very closely watched and analyzed that rebellion, whether in rule only or against the entire system, is a life-threatening risk. Julia knows this, and yet rebels against the system anyway. She has spent quite a bit of time rebelling against it, and it has improved her quality of life while she has. Unfortunately, getting caught will mean the end of any form of independent living or thought whatsoever. Yet, it is still worth the risk to her.
So, because of this, I don't find her rebellion insignificant. Anything that one endeavors to do at the risk of your life is not a light-hearted, flippant thing. Julia, although maybe not rebelling against an entire system, sure has a hatred for all that system does, and risks her rebellion at the cost of great sacrifice. I hope that those thoughts helped; good luck!
No, Julia's rebellion is no less significant than Winston's. This is because Julia's actions would be just as devastating to the Party and the society if more people emulated them.
There is, after all, a reason why there is a Junior Anti-Sex League. The Party does not want people forming romantic relationships because it will give them someone other than the Party and Big Brother to be loyal to. So if everyone went around having affairs, it might lead to less loyalty to the Party.
In addition, Julia's actions are disobedient. Any disobedience is significant in a society where every action is monitored. If more people were disobedient, the hold that the Party has over the society would be disrupted.
Finally, her rebellion is about being an individual. She does things because she wants to. This kind of individuality and desire to have a good time is disruptive as well because the society of 1984 puts a great deal of importance on being just like everyone else and not wanting things for yourself.