I'm actually in disagreement with my fellow moderators here. The metric system is used by an overwhelming majority of the world's population. As said, both the Internet and amateur radio are inherently international in nature. Even in non-SI countries, SI units are frequently used (consider Hz and multiples thereof, or the measures of time which are multiples of seconds), and even US customary units are defined in terms of SI units. They are commonly used in electronics and technology, both of which relate closely to amateur radio. They have some significant advantages, not the least of which being that they often simplify calculations.
I'm not necessarily saying that we should encourage edits simply for converting from one system of units to another; good arguments have been made why edits making only such changes should probably be considered to be too minor. (Though if someone makes edits that significantly improve a post in other ways and also happen to include a conversion to the other system of units, I certainly wouldn't edit those conversions out of the edit.)
What I am saying is that we should consider the international reach of this site. We already have a number of members from all over the world, including (just to name a few) myself (Sweden), the OP (Belgium), VU2NHW (India), Pedja YT9TP (Serbia) and probably plenty more. Many more non-US visitors are likely to be familiar with the SI units than are familiar with US customary units. The number of people familiar with both systems of units would be even smaller, and my gut feeling is that this group would largely consist of certain portions of the population in the three major countries (US, UK, Canada) that to a lesser or greater extent use US customary units.
Yes, conversion factors are easy to come by. But if our goal is to build a collaboratively written and edited collection of knowledge on matters relating to amateur radio, I think it's unfair to say about such a simple thing as conversion factors between different systems of units one of which is used by something like 90-95% of the world's population that people can "just search for it".
My personal feeling of using only SI units is probably not realistic due to the admittedly large fraction of users here particularly from the US, so as a compromise, I propose the following:
Encourage use of the SI system of units, but consider edits only making such conversions to be too minor for the reasons stated in other answers. Encourage posters and/or editors to make such conversions while they are improving posts in other ways, such that posts ideally include figures using both systems of units. There will be a period of time before this catches on, but while I'm not sure exactly how to make such encouragements, the volume of posts is still relatively small so it shouldn't be too difficult to establish clear precedent in the matter.
Publish conversion formulas and/or tables (as appropriate) in the site FAQ or as a canonical question that can be referenced widely. We do have MathJax support ($\LaTeX{}$) which is perfect for such things.
I suppose an exception about the "edit is too minor" guideline could be made for posts that are already near the top of the front page, as bumping those again won't have significant impact on the visibility of them or other posts, but if we do that, it comes down very much to personal judgement of the person reviewing the edit, and personal judgement is always slightly iffy.
