So many opinions indeed! Mine: heck yes you'll notice them. Can you still have a great vacation? Definitely Yes.
I've been in late May 3 different trips over grad nights so these are my experiences. Best strategy: DL during the day, DCA at night. We've tried braving DL on a few different occasions on actual grad nites each trip (because we're gluttons for punishment I guess) and it never went well. The first time we FP Indy at like 10pm and besides the cue being full of hyped up teenagers that late at night Adventureland was more of a traffic jam than usual. The second time we just went in to ride Haunted Mansion around 11pm and it was actually ok, well there were a lot of extra screams from say every other buggy but none were rude or obscene. Last trip we decided we wanted to eat at Plaza Inn for dinner, see fireworks and PTN and to this day I don't know WHAT we were thinking. It was PACKED and the kids were rude and the families were upset with the seniors and each other and it was not a happy place. We grabbed a table and planted ourselves there for the next few hours until the late PTN finished. Yeah, not our best night...
Generally speaking you start to notice Seniors in DL around lunch. They aren't annoying or rude at this stage generally they're just starting their days and there really aren't too many. We always exit the park a couple hours later and return around dinner. The grads are much more noticeable at this point. Now you should avoid the "popular" DL rides (Indy, Space) because this is the grads last chance to hit the rides before being pushed out to DCA later that night. Around 8pm crowds in DL get really thick and the kids are getting rowdier by the hour. I don't blame them, they're getting closer to their party and there are so many other seniors around by then but as an adult I don't care for the atmosphere by then.
The key: Park Hoppers = DL during the day, DCA at night. DL up until dinner if you must. After dinner time in DL on a Grad Nite? Don't be me! DCA is your friend that night. (The party is in DCA after it closes to the public so most of the kids are trying to do eveything they can in DL up until the parks close and they're forced into DCA.)
I would never advise someone NOT to go to DL during grad nites. But do prepare (like you are!) so you know when to avoid what areas and you can have an entirely pleasant trip!