This article - http://stroke.ahajournals.org/content/26/4/562.full - a population study at high altitude - both suggests, and provides a bit of basis from another study, that high altitude is helpful and not a risk factor at all in strokes. The incidence of stroke at lower altitude was much higher.
Being a stroke survivor myself, and now living at relatively higher altitude (7000') I am extremely interested in this as well.
The altitude of Quito gave my husband such frequent, severe nosebleeds that we had to descend much earlier than we intended. Now that he has had a hemorraghic stroke (10 years after being in Quito--no connection) I wouldn't dream of taking him to a high altitude. I haven't read anything about this, just my intuition says "NO".
Good question! I've been able to find only i study about this that suggests traveling in and being active in altitudes of 8500' or higher can lead to more cardiovascular problems. I'd like to know more. I've been a skier for over 30 years and altough I'm physically able, I'm concerned about the effects of the high altitude.