First something probably dumb, just to confirm that connecting the positive terminals of the battery and a switch source will have no effect on each other?
And the actual question, on a bike would a relay protected by an in-built resistor be fine or would it need a diode?
The background, incidentally, is to use a relay to enable a light switch to power some LEDs from the battery only when the bike is powered. However if the lights are already on when the bike is turned off, the lights should stay on until the switch is opened.
So the battery goes to 30 and 87 to the lights. The switched connection goes to 86, with 85 to ground via a switch. But 87 is also connected to 86, so that once the coil is engaged from the switched source it will be powered by the battery until the circuit is broken.