I'd have to say that all the Runco projectors out there, with the exception of their new high brightness LS-HB, are really intended for dedicated theaters, or at least rooms that are well set up for cinema viewing. As such, screen selection is likely fairly simple. We know the brightness, so we know that the LS-5 should do very well with traditional 1.0 to 1.5 gain screens of fairly large size (though no match for their 3 chip projectors). I've been watching the LS-5 in my unfinished but serious home theater - black ceiling dark navy blue walls and dark blue carpet. Movie watching at about a 130" diagonal size looks great in my room, brightness wise. (My screen is a 124" 2.35:1 shape). I don't use an anamporphic lens, but with the LS-5 fairly far back, I can fill the screen (the letterboxing at top and bottom are hitting my dark blue wall and are invisible), with the movie image filling the whole screen. (For 16:9 viewing I have to move the Runco projector closer.)
I use a Studiotek 130, which is considered the industry standard by many (check out those movie theaters). It has a 1.3 gain. Some might want a gray screen for some purposes. One reason might be if they want some sconces on the sides of the room to provide a touch of consistent lighting. A HC gray screen will reject a lot of that side lighting, thus not impacting the image. Otherwise though, I'd say a standard white surface is the best choice. I'm not a big fan of high gain screens, like 1.8 or 2.5, because the evenness of brightness suffers (corners are dark, etc.)
Let's put it this way, choose the screen that best matches your room conditions and viewing requirements.