To me, denim and a sports coat is the same level of dressiness as slacks with neither jacket nor tie (assuming shirt and shoes are of similar formality). Adding a tie (or vest/sweater/pocket square) does nothing for formality - wear one if you wish but recognize that it's a stylistic accessory only.
As for the dos/don'ts, there are far too many to list but the top 10, for me, are:
-make sure both fit impeccably
-jeans should be at least as dark as the jacket, a black jacket being the sole exception
-jacket should be two-button, single breasted only (and at least 80% of the time, notch lapel)
-stripes are only OK if it's a really thick chalk stripe with an almost fuzzy-looking flannel material (even if it is from a suit, it's a less formal suit and doesn't scream "1/2 suit")
-orphaned/borrowed suit jackets are alright if the material is sufficiently interesting (NOT the same as busy) such as a large, bold windowpane, a glen plaid or a plain fabric with a unique texture
-avoid polished oxfords; scuffed ones are great
-avoid sneakers (95% of the time, a couple good situational/stylistic exceptions)
-don't use shirts that would generally look out of place with jeans (like stuff ripped from the JAB catalog) and don't get too colorful here, solids and simple stripes/checks are best; square/tie for color
-typically avoid dark, cold colors; the whole thing should look warm, rich and inviting (certain black outfits are exception but difficult to pull off and 99% of the time avoid black/grey for the shirt)
-a Chelsea boot is (to me) ideal footwear but a slip-on or monk strap has its place as well
The overall goal is to avoid looking like a guy who bought a lot of expensive suits in the 90s and now either works for himself or whose office has gone casual so comes in wearing relaxed, too light jeans, a three-button, too-long, too-wide triple pinstripe low-notch suit coat atop typical "dad" weekend shirts such as baggy OCBDs or, worse, logo-displaying golf shirts and black New Balance or gold horse bit Ferragamos. Get as far as you can from this and you'll be fine.