It is a gamble, and I think that's the appropriate word. There are two outcomes: either the RO says "no," and you have to wait in the sometimes-hours-long stand-by line, or you're given the go and you end up saving 60-120 minutes of standing up in dry heat. Having the courage to ask is something I advocate, and it can save you HOURS; click on the link in my siggy and check out the "data" section.
All the RO can say is "no," and you should respect their decision.
Rides at Magic Mountain that have posted single-rider lines (something we call "SPRQ", pronounced "spark") are:
Green Lantern: First Flight
Lex Luthor: Drop of Doom
Revolution
Riddler's Revenge
Scream
Viper
However, Roaring Rapids regularly seats single riders (ask), and both Superman and Goliath are known to admit single riders. Always ask.
Again, check out our "data" section and click on "October 11, 2014." That was a very busy Saturday: Goliath had a 120-minute wait and X2 was busy through the late evening. "S" stands for single-rider, "R" means stand-by queue. Look at how much time was saved.