I have done this four times (two relatives, two friends).
I find it really helps. For me, it's a sort of closure, something definite. It helps me to get my head round the fact that the person has gone.
This I found especially with my Nan. The first body I'd seen, we did the rosary vigil. It was really good to see her at peace, as the last time I'd seen her she was agitated in hospital. I won't go into details but she'd been less and less herself and then very poorly. Quite honestly, I think I was motivated by the morbid curiosity of seeing a dead person, but seeing her at rest was more healing to me than I could have possibly imagined.
With my maternal grandmother more recently I sat with her in the chapel of rest and chatted. I touched her body and kissed her forehead. I was aware that the egg that became me was formed in her body (with my mum), and as a mum of sons with a sister who adopted her children, that that line has ended.
My mum had been in previously and found it similarly healing.
My sister refused to go. My dad hates the idea.
They don't want the last memory they have of someone being a cold, dead body.
There is no right or wrong way, and I think people instinctively know whether it will be helpful or not. So if you are tempted to visit a loved one, then do.