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How important is temperature control for yeast starters? Obviously you don't want to cook it, and if it's too cold it won't do anything, but so long as it's in a range where it can grow, is it generally ok?

If I'm going to be pitching the starter wort along with the yeast, I'm rigorous with temperature control. If I'm not pitching the starter wort, I still try to keep the temps sensible, but don't worry too much. Any crazy flavours the yeast chucks from being too warm are going to get decanted off, right?

Wyeast smack packs all come in the same packaging, bearing the same instructions, suggesting that even lager yeasts should be activated and grown at 21-24c. Would it be ok to, for example, grow a 2L starter of lager yeast at 21-24c, chill, decant and pitch into a 5L starter at 10c, all of which will be pitched into the main batch at 10c?

Cheers guys!

What you propose sounds fine to me.
The objective of a starter is to grow yeast. They grow much happier at a slightly higher temperature than we would normally ferment at. I don't think the temperatures you mention are high enough to cause any change in the character of the yeast, and I'm sure they'll perform just fine if you grow them that way.
I pour off the starter beer, so as far as I'm concerned I see no problem. I don't use temperature control on my starters, except when they're fully grown. Then I'll chill them to my desired fermentation temperature, and pitch the slurry to match my wort in the fermenter.