This book appears to have a puzzle with only 18 clues that is rotationally symmetrical.

7 2 . . . . . . .
. 5 . . . 9 . . .
. . . . 3 8 . . .
. . . 4 . . 5 . .
. . 3 . . . 9 . .
. . 1 . . 3 . . .
. . . 2 5 . . . .
. . . 6 . . . 3 .
. . . . . . . 1 9

But I still don't know if a rotationally symmetrical 17 exists.

This book appears to have a puzzle with only 18 clues that is rotationally symmetrical.

7 2 . | . . . | . . .
. 5 . | . . 9 | . . .
. . . | . 3 8 | . . .
------+-------+-------
. . . | 4 . . | 5 . .
. . 3 | . . . | 9 . .
. . 1 | . . 3 | . . .
------+-------+-------
. . . | 2 5 . | . . .
. . . | 6 . . | . 3 .
. . . | . . . | . 1 9

But I still don't know if a rotationally symmetrical 17 exists.