IMHO no store employee should be touching your phone unless you explicitly instruct them to. If the carriers really think a task killer should be on there, it should be tested and baked into the ROM before the phone ever reaches the store. There should be a corporate policy against employees doing this at the store.
For those of you wondering why I am so passionate about this, it's because 80% of the "I'm having an issue, please help" threads I've been monitoring since day 1 are almost always related to task killers. If this wasn't the case, this thread wouldn't exist. Not to mention it makes me uneasy to think of employees installing software before they hand it to you.