I have KDE panel on top of screen so I want to have krunner show on bottom of screen. Currently it's impossible. Reproducible: Always
Alt+F2 > Configure wrench > User interface > Free floating window > Apply. Then you can use Alt+mouse to move the window anywhere, including to the bottom. From what I quickly checked, it remembers this position between invokations. If this resolves your issue, please close.
Try to type something, krunner will expand its window and next time it won't show on bottom but in last remembered position.
Usability: Do we want this feature? Given you could also add the Search applet to your bottom panel, if you like.
I agree that the problem can easily be solved by using the Search widget instead, which can be positioned freely. Currently, however, the widget does not provide all features that KRunner does (see bug 360359). It makes more sense to bring the widget to feature parity with KRunner than allowing to change the positioning of KRunner, so I'll close this bug here as wontfix anyway.