If you're buying new brake lines, definitely upgrade to stainless. As far as rebuilding the calipers, I rarely do it, right or wrong. However, every time I change brake pads, or about once a year whichever is first, when I remove the calipers and remove the pads, I gently pump the lever to push the calipers out a bit (doing one side at a time), then clean with simple green, rinse, use a bit of brake cleaner (not too much, claim it can damage the rubber seals), dry with compressed air. I think routinely keeping them clean helps.
That being said, I did finally rebuild the front calipers on my '07 gsxr track-only bike. One piston was stuck, so I figured it was time. I did have to rebuild the rear master cylinder on my '01 cbr 929 when it was about 10 years old because it stuck.
It can be a pain to get the pistons out when there's more than one. Compressed air works, but, of course, one of 4 on my gixxer popped out only. I purchased a caliper puller tool, worked great for removing the pistons.
I'd say clean them first, and ensure both move when you gently pump the lever. If any are stuck, or you're getting uneven pad wear, it may be time.