Harmony styles I use frequently:
1-up - Everly Brothers
2-up - Beach Boys
1up-1dwn - Eagles
8va up - (falsetto)
8va up & 1up-1dwn - 4 seasons
8va dwn - Barry White/Darth Vaderish - comic relief

Sometimes, all you need is one above - other times, you need that fuller, up and down choir effect ... and everything in between.
If I do "Let's Hang On," or "Can't Take My Eyes Off Of You" (4 Seasons) I need the octave up, (plus a full vocal group above and below my lead note to get the most realistic affect. Same thing for "Betcha By Golly Wow" ... I need that 8va blend to get me out of the basement.

so ....... yes, the people notice.