Ok but what about in the situation I describe? At home, I tune it. In the field, I put the antenna at about the same height. Would it be sufficient to say the tuner should be placed in roughly the same spot for both locations, or can they really vary that much?
Depending on the antenna resonance, e.g. if it is a half wave dipole, and thus quite broad band, then YES in general. And it won't do damage to the Pixie even if you transmit without an antenna (I THINK!). I've used a Pixie for 3000km QSOs but the problem was not just that my signal was very weak, I think it only put out some 350mW or so, on 7023kHz, but also that the receiver seemed very deaf! I suppose with so few components you cannot expect a sensitive receiver! Wide as a barn door doesn't bother me, I have filters in my brain-ears, but lack of sensitivity does. Other stations that were S9 were just copyable. If they had ALSO been using Pixies, no way would the 2-way long distance CW communication have worked. A pixie is a pretty poor choice. But that's just $5 including postage!
Here are good, I hear, QRP 1 Watters and there is a 5 W amplifier there too: they are single band TX/RX but tune a few kHz, I'm thinking to try one:
http://www.kitsandparts.com/1watter-V3.php -- at $47 each doesn't it seem a good investment to buy one for each band? If one goes missing, gets squashed, hit by a fly or gets stolen instead of your jewelry, then you are at least not QRT!