Don't know why they would effect the cruise control, but you might need to add resistance so the turn signal flasher works correctly.
On my fiver there are two taillights on each side. When I put LEDs in all four it was not enough load, so I put incandescents back in one pair. Now my top pair come on a second or so faster than the bottom pair, both signal and brake lights. Figure that can't hurt since it is attention getting and the flasher likes it.
I read once that at 60 MPH the quicker activation of LED brake lights is worth 30 feet of stopping distance for the driver behind you.