I'd only consider myself at a very modest level of knowledge and experience with both these substances, but hopefully you find this information useful, lol.
Valium has a very long half life compared to pretty much all other benzos (200 hours approx.), though the immediate effects will subside much sooner. If it's only a slight dose of benzo, I think the main thing you'd really notice is an enhancement of the anxiolysis/relaxation. This is really good if you're naturally anxiety prone (like I am, lol).
As I'm sure we all know, the mental state that a substance induces is a main factor in the kind of trip that you experience. So if you get rising anxiety from lsd, like many people do, it could easily lead to a tense experience where the drug might push you into addressing important psychological or other personal issues. While I agree with most of the posts here, I don't necessarily think benzos before a trip is a bad idea at all, lol. I wish I would have used benzos during my first mushroom trips since anxiety can actually stifle the effects of psychedelics which left me kinda let down and frustrated for the first handful of experiences.
IME, the main thing that benzos reduce is the hallucinatory/visual effects of psychs. The mental effects on the other hand are subjectively altered if there's even any change at all. Maybe the focus is just switched over to the person's train of thought, although this is just a speculation. I actually had a really awesome experience using 4-aco-dmt with both adderall and klonopin (although this trip was EXTREMELY visual and I got lost in a dali painting that I had as my desktop background for a half hour or so). I'm actually prescribed to psychiatric medications, though, which may have been a factor. Unfortunately, all I can do is speculate on the effects of a combination with lsd since all the L I've used has always only been with high grade cannabis.
Anyway, hope this helps and happy bicycle day and 420 everyone! lol