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This question came up a while ago. The input focal length is mostly just used by the camera for shake reduction and EXIF info. I can't speak for the K10D, but I have a K200D which is fairly similar.
If you change the lens while the camera is powered on, you won't get this dialogue again. However, whenever you power the camera on (with SR engaged) with a manual lens, you will get it. For prime lenses with a focal length not listed, you select the closest available focal length. For zooms, there is not so much certainty as to what FL to set. If you know in advance what range on the zoom you plan to be using, you should set it in the middle of that. Somebody did a small study on this and found that it is best to underestimate the focal length (as opposed to overestimating). This of course holds in theory as setting a shorter focal length implies that the sensor moves less as opposed to more. So, if you are really, really unsure about which FL you'll be using on a zoom, set the IFL for the bottom end - it won't make your pictures any worse than sans-SR.
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I almost forgot - if you need to change the input focal length at any time, it is the very last option on the capture tab of the menu so just hit menu->up->OK and find your FL.
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I hope this answers your question,
-Al
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