There's a misconception about LEDs and relays that I hope to clear up here:
Because LED bulbs use less power (less amps), an *electronic* flashing relay on the fuse panel may or may not think that the bulb is burnt out and may double the flashing rate. You can buy a relay (or modify your existing relay) to compensate for the flashing rate. Try the bulbs in your car before wasting $40 on a relay that you may not need. This obviously won't work on thermal relays - probably time to upgrade those.
Any given LED bulb needs a resistor. The resistor needs to go on the power circuit that powers the bulb (not to the line that powers the switch). Many automotive LEDs are plug-and-play, but you may have to add a resistor if the bulb doesn't power up. You'll have to match an appropriately sized resistor to the power rating of your bulb. Don't guess.
LEDs have a power threshold for turning on and off, e.g. 2 or 3 volts. Thats why they turn on and off crisply, while normal incandescent bulbs fade in and fade out. There's no reason to avoid adding as many LED accessories as you want. As with any circuit, you have to add up the amperage draw of your accessories and stay below the circuit/fuse limit. There's nothing to fear about damaging old car relays or switches - they simply provide power to LEDs that will try to use that power.
Enjoy your LEDs.