The key thing is targeting specific dragons based on your current power level. Start with elder dragon greyoll in caelid, since she's stationary and drops 5 hearts plus massive runes. Getting to fort faroth first makes this easier, and you can safely attack her tail while mounted. For early progression, flying dragon agheel near dragon-burnt ruins is manageable, especially if you stay on horseback and watch for flame attacks.

Dragon hearts are valuable regardless of build choice since dragon communion spells/incantations like rotten breath remain effective even with minimal stat investment. The cathedral of dragon communion in caelid lets you trade hearts for stronger incantations, while the basic church in limgrave works for early game exchanges.

Tip: even without using dragon powers, the rune rewards from these fights make them worth pursuing especially greyoll who gives around 70k runes. Focus on mounted combat, hit their wings or tail when possible, and keep some distance until you spot clear attack openings. Later bosses like great wyrm theodorix offer better rewards but need more careful timing and positioning, so maybe save those for when your build is more settled.