The best place to discuss questions related to grafting fruit trees, IMO, would be the Fruit and Orchards Forum. If you do a search of that forum, you will find many many threads full of information about this topic.

To answer your question about whether grafting less-suited scions onto well-suited rootstock, it would depend entirely on the specifics. There is no simple answer to the question. Many rootstocks are chosen primarily for their ability to produce trees well suited to certain soil or environmental conditions hostile to potential scion cultivars. On the other hand, some environmental limitations could not be overcome merely by rootstock selection.

Since you mentioned terminology, I'll give you this tip. Use cultivar to describe different named selections of plants typically propagated asexually, and use variety to describe a genetic group of plants containing similar and inheritable traits, that vary from other members of their species, known to come true-to-type when reproduced sexually. In other words, the term variety, technically, is more akin to subspecies than cultivar. Misuse of these terms is very common, and I'm sure I do it quite often myself. It can lead to confusion though, so is best avoided, if possible. Other than that, you sound like a pro.

Thanks for the info Brandon7 on Cultivar and variety. I guess I never picked up on the difference since like you say everyone uses them interchangeably. I have seen the same tree called cultivar and variety from different sources. I will revise my question to a more specific situation and post it in the fruit forum when I have some extra time next week, after I do some searching.

thanks,
Craig