I used to level up only my starter when I was younger, but now I train my team to be within 2-3 levels of each other. What I would usually do to make the leveling feel like it is going faster is to train the one that is closest to the next level, and then go down the line from there.
For example, if Togekiss has 10,000 exp left to get to the next level, and the others have 13,000 or more, I do Togekiss first. It just makes it feel like it's going faster instead of just randomly trying to make them all the same level. I know this has to do with how many total exp points each pokemon has, but I guess it's psychosomatic.
I hate having one pokemon who is a much higher level than the others. If I were to level up my Meganium to be so much higher than the rest, and I encounter a fire, flying, or poison pokemon, I'd be screwed. One boy in middle school looked at my Sapphire team and said, "oh they're all the same level; that's bad. You should have one pokemon be higher than the rest." He ended up being a junkie. :p