Is there a trick to casting off in the round smoothly and not having any noticable bump or space between the first and last cast off stitch?
I usually just thread the tail left at the end of the cast off onto a needle, then sew it through the first cast off stitch. It usually closes the gap and gives a smooth ending.
Abi
Thanks. Almost sounds too easy. 
This might not be kosher with out knitting purists, but after I get to the “last” stitch in the round, I pick up one extra stitch from the cast-off edge of the first stitch, and then cast that one off as well… THEN run the tail through and all that. I don’t know if that makes the join neater or anything, but I picked that up from another knitting friend, so that’s what I do.
Thanks for another option. When I’m done with this sweater I’ll be dancing and singing. 
Yet another option…
I use a crochet hook and single crochet the last stitch to the first stitch of the round, then bring the yarn tail through the stitch and weave in the end. I think it smooths out that last stitch nicely.