Is it safe to keep lightning cable plugged into iMac ?

As long as you're not bending the cable at an acute angle near the connector (which could damage the cable), it shouldn't be a problem. The only other issue I can think of is that if somehow the cable gets yanked on, it may not come out, and the computer could be pulled. If it's close to the edge of a desk or table, that could be bad but that seems like more of a MacBook problem. Don't leave it dangling were pets or small children can get at it.

Yes.... I do that with with an iPhone lighting sync cable and a USB drive cable for a couple of reasons.

1) So I do not have to fumble around plugging and unplugging cables.

2) To help prevent needless ware & tare on the Mac's ports.