Welcome, i_j_r,
I'm not sure you'll find a lot of love for jeans as 'smart casual' on this forum. But as a native Texan who's been wearing them since age ten in the mid-60s, I suppose some diplomatic advocacy is in order.
For wear with a jacket you might think carefully about the low-slung cut. As Michael has pointed out in his "Illustration of the week #17," a low rise shortens the legs and lengthens the torso; more to the point it makes it likely your shirt will show sloppily below the button point of your jacket. That look may be fashionable at present but it can't be called elegant. I notice that Daniel Craig wears jeans with the vest and jacket of a suit in interviews on the "Casino Royale' Blu-ray disc and doesn't look bad, though the rise is high enough for the vest to cover the shirt even when he's sitting on a stool.
Boot cut is up to you. I can't help with exotic and designer brands, including the various fanatical Japanese reproductions of Levi's from specific eras made on the original looms with ring-spun yarns (but not the trademarked arcuate pocket stitching), or the artfully hand-distressed (signed by the distressors, yet) versions for the impatient. They're all gilding the lily, in my view.
Last year I purchased my first shrink-to-fit 501s in over a decade, and while the denim is less robust than when I first began wearing them, it is not catastrophically so. My understanding is that the old looms which wove the ring-spun denim were moved to Japan in the 1970s, so I've presumably worn jeans from both the old and new cloth.
What I can clearly say is that, during my lifetime, I do not recall any 501s having nearly the exaggerated variation in warp thickness, or the degree of streaking that I understand is called "vertical fall" by denim connoisseurs, which seem so highly prized now. I presume, if this is a true "vintage" characteristic, it harks back to the dungarees made up through the immediate postwar era. Growing up in San Antonio, most all the ranch hands and working people I knew wore 501s (a few wore Lee Riders or Wranglers) and those jeans had a fairly even weave and color, in which the diagonal twill pattern was more prominent than the warp. Those are the jeans that became counterculture icons in the '60s, worn over western boots, Clark's desert boots, or with sandals--or in the fashion Warhol started, with Weejuns and a navy blazer.
I like them because I'm used to the cut (high crotch and roomy hips, which flap a bit on us skillet-assed guys when standing, but are comfortable when sitting a horse or an Aeron chair) and because they're plenty sturdy for the amount of heavy use I give them now. I usually buy four pair at a time, and they last in rotation for ten years or so. I also find it distasteful to spend $250 on what should be workingman's pants, even though I have no trouble spending more to have trousers made. I wish STF 501s were still made here in the States, and I'd gladly pay more than I do now for them if they were, but so long as the cut doesn't change much and the cloth is decent, I'll buy them. In my view jeans are the wrong kind of garment to turn into a rarity fetish.
I have heard a rumor that Levi Strauss is planning to drop the reinforced, felled inseam on new production STF 501s next year. If they do, that might shake my loyalty when next I'm in the market. I mean, saddle wear is pretty hard on that seam.
The 501 cut was designed so that it could be worn either outside (a bit snug) or inside your boot tops without a lot of extra cloth in the way. There are plenty of times when you'd rather have leather exposed to the mesquite thorns and limestone outcrops (or thistle and gorse and such if you're riding in Britain) than your trouser legs, so working cowboys often tuck 'em in. Besides (whatever happened to the old pictorial thread on well-used shoes?) if you have boots that have earned this kind of dignity, why would you cover 'em up?
(A hand on the Pitchfork Ranch in the Texas panhandle, from a 1980 Pulitzer Prize-winning photo essay by Skeeter Hagler)
If the slim legs bother you, just pretend they're Neapolitan.
Good luck!
