I disagree. You can try to ignore it, but they're still part of the same story.
I can ignore Rob Zombie's crap Halloween series because it is a reboot with different actors. If Jamie Lee Curtis wasn't in Ressurection, I could ignore that too. But she and her character was. Would have been great if they left it with H20. It was the perfect ending. But like it or not, the story got continued and so H20's ending is no longer an ending really.
I still think H20 was a worthy sequel, but the ending doesn't have the same effect now that we know what happens next.
Also, I hated Alien 3. That too made the Aliens ending lose its impact because we know how the characters end up.