Regardless of using a direct tie or swivel to attach a leader the correct knot is a must.

Generally a quality swivel is recommended for attaching leaders. Much easier for a re-tie (especially in low light conditions) than re-tieing direct plus helps eliminate leader twist.

I run 8'6" rods with 8 to 10' leader so I use a no.12 swivel that fits through the eyes without causing damage.

I use a backsplice to attach the swivels to the leadcore and even than, I'm all thumbs tieing on leaders and, I still don't catch any stupid fish.

See YouTube for info on tieing knots using leadcore.

Have fun and tight lines.

Anyone ever try the Manns stretch lures.  They are designed to dive to deep depths without leadcore or down riggers.  They have them designed to run at 25', 30', 40' and 50'.  The 50' ones are crazy expensive ($90), but the others are usually under $20.