Like some have said...if I can hear the individual piano tracks that are more than say, just R&R chords getting pounded...
...yeah, I can tell the difference.
In a busy mix with more subtle parts, not as much if I'm listening to someone else's stuff...but when I use sims or electric pianos in my own projects, and I'm looking for a real acoustic piano sound...I can hear it then too, it actually bugs me sometimes. No matter how much I try to mask it in the mix, I can tell it's not an acoustic piano sound, if that's what I wanted. Mostly it's the higher octaves and the tails.
I've tried a bunchi of sims and samples, electric pianos, and I've had an upright piano in the studio for years...and with each new songs recording that needed a piano, I would end up going through them to find which had the most authentic sound in the mix. I liked my upright, though it never had that really pure tone, and always a PITA to have to tune for recordings.
I know have a small grand in my new studio that has that pure tone sound only a grand can do, and I look forward to recording it...but I may still end up with some sim/sampled piano on some things, because again...sometimes it's easier, and no tuning needed if I'm in a hurry.
I even got another electric piano...since my old Kurzweil was having issues with some of the keys, and I got tired of fixing it...but it had a couple of pretty good piano sample. Now I have a Yamaha, and it's pretty good...but I haven't really taken the time yet to sit and compare the tones to my other choices, or to the grand piano.
There are also times when you don't want the big grand piano sound...like when you're banging R&R chords...
...it can be too much for a mix, and an electric might just sound better. I've got a decent piano library from UVI...and I also have all the Toontrack keyboard stuff...so plenty of choices.