Standard relays are load dependant. This means it relies on a load to flash at its normal rate. If you put a led in you remove the load and so the system will flash faster than it should (the same will happen if a bulb blows). The best idea is to get a compatable relay that is made for use with leds. These relays do not look for a load and will flash normally. A comment was made earlier that leds last longer. Well they may, but I do know from experience that they also fail now and again. Bulbs will last a long time. I own a VR and alot of the bulbs on the car are stock from when the car was built (12 years ago). Thats a long time really. Why would it matter if leds are going to last so many hundred thousand hours, you will never get that much use from them. Unfortunatly leds do sometimes fail after just a few hours of use and it can be a real pain. Not that I have anything against them, i think they are great if setup properly. If installing them in the dash try and get some that have a wide viewing angle. Most manufacturers will give this to you in their specifications. Otherwise you will get a really bright spot in one place but dull in another. Or you can cover it with a bit of wax paper to disperse the light.
kopper69