While no amount of high-end gear will make you good at skating without practice and experience, a well-functioning setup will help good riders skate better. And just as doing long standup slides on a straight hill doesn't mean you're good at slowing down and taking good lines through corners, being good at skating doesn’t mean you necessarily know how to build a good setup. Selecting parts to create a board that balances turning, grip, and stability is a skill separate from actual skating.
I may not be the fastest racer or the most stylish freeride guy, but thanks to several years of heavy Silverfish use and a little real world experience I’m pretty good at wrenching on boards and making them ride good.
I can say from personal experience and observing my friends’ skating that sleeved flat bushing washers are fucking garbage and nobody should ever use them for freeriding or downhill.
Allow me to explain. We'll start with the basics of how bushing washers affect skateboard turning.
Flat washers allow for ample deformation of the bushing, enabling deep lean and turning. This is very useful for getting short bushing trucks like Paris, Caliber, and Aeras to turn deeply.
Cupped washers restrict the bushing’s ability to deform, creating additional resistance as you lean farther and the bushing binds in the cup, eventually creating a sort of stopping point at which the truck will not turn any more. This aids stability and can be useful in preventing wheelbite. (Cup washers are also crucial for tuning tall bushing trucks, but that’s another discussion.)
Sleeved washers don't restrict turning like cup washers or allow for ample bushing deformation like flat washers. Instead, they allow for deep turning and increase rebound as you lean, giving your skateboard a snappier return to center. This is good for when you want a lively, bouncy turning response or when you’re riding low-rebound bushings. It is really really bad for downhill and freeriding.
To have any kind of stability, a downhill board needs to be able to lean over and stay leaned over. Too much rebound in your bushing/washer setup will make your board want to dig in and Michael Jackson-style high side on stand up sides, wobble out of predrifts, and throw you off your line through gripping corners. These are not performance characteristics that I find helpful when I am trying to stay in my lane and on my skateboard at 40mph.
Now, you may be saying to yourself “but Dubler, I have these washers in my board and I can skate downhill just fine” or "I like riding a squirrelly and unstable setup because it's more exciting." That may be so, but you would skate better and more confidently if you weren’t battling a poorly-designed setup so shut your dumb mouth and switch your sleeved washers out for normal ones. You will almost certainly notice improved stability, easier cornering, smoother sliding, and less twitchiness.
(And while I'm hating on these things, let's get one thing straight: machined sleeve washers do not make your trucks "more precise" in any meaningful way, and they certainly won't make your cheap Chinese cruiser trucks ride like high-performance racing trucks. A precise truck has perfectly straight axles with machined hangar shoulders, precision-machined pivots with tight pivot cups, and precision-machined bushing seats. Bushing-on-kingpin slop is not really a thing. If you want to make your cast trucks perform better, invest in urethane pivot cups and good bushings, preferably Venom because they are the best and they give me money.)
TLDR: sleeved bushing washers are fucking terrible for downhill and freeriding and you should not use them.