We were about a half hour or so after the starting time for an eride night last summer and it was fine. We just used our tickets and got our wristbands with absolutely no wait at all.
If you want to take full advantage of the eride, I would be there about a half hour early or so, get the wristband and start on the rides. You can usually get one or two in during Spectromagic with very short waits!