You can do it, but it does change spectrum and can reduce the bulb life. Most of the well known brand ballasts when dimmables came out had it written in the manual, saying bulbs should be swapped out for the appropriate wattage your ballast is running.
However things do change, I mean your lucky to get a manual of any sort these days. But it does seem that its easier to just not bring this point up since its better for the manufacturer, the product sounds better without the info and they all have their hands in the cheap bulb market too.