the guy at my local lfs REALLY made it sound like sump will be needed
Yes also on the LFS advice, a sump is not required. Others here have already covered, they increase volume, give you a place to hide and add additional equipment if wanted, and they do add some complexity.
As noted this is our second 55 G, (second one above) here's a pic of the first 55 (only broke it down to move 5,000 miles) running live rock/sand, an HOB protein skimmer, no sump.



