How about when leveling? (if you have to level alone)
You do need to reach Act 5 hell and level your Smiter to go do the ubers, so I think an Act 1 or 3 merc would help with range support. Probably Act1 are the best since you're gonna have Fanaticism anyway. Put a Kuko for lower difficulties (normal and NM) on a cold rogue, and you're be glad u did!
Not sure how the rogue would do in Uber... never tried it, but she shouldn't attract too much attention, she'll just keep you some company