> This stems from Heraldic terms
Not really. Dexter and sinister were simply right and left in Latin, respectively. Heraldry took most of its terms from Old French (which some people call Norman), since that was where chivalry originated. Chivalry originally meant simply the occupation of being a 'chevalier', which we call in English a 'knight'.
In heraldic terms, dexter means on or starting from the wearer's right, and sinister the equivalent on or starting from the left - no notion of good and bad implied. The heraldic adjectives are dextral and sinstral, not dexterous and sinister.
However, quite independent from heraldry, right-hand magic was always good and left-hand magic was always bad. Why this was is not fully understood. Therefore, via standard etymology, dexterous has inherited positive association and sinister bad.