Below is from Jamil Zainasheff’s article Making a Starter (Zymurgy March/April 2007).
Q: IF I’M MAKING A HIGH GRAVITY BEER, SHOULDN’T I MAKE A HIGH GRAVITY STARTER SO THE YEAST BECOME ACCLIMATED?
No. In general, starter wort should be between 1.020 and 1.040 (5–10 °P). Lower gravity starters are easier on the yeast, but result in less growth. High gravity starters result in more growth, but are more stressful for the yeast. Logsdon says, “Generally, you’d use the lower end of that range [1.020 SG, 5 °P] for coming off a plate or slant or very old yeast. Yeast don’t get used to a high gravity environment, and the high osmotic pressure can really stress the yeast.”
I would recommend that you download the article; lots of good information in there.