Whether you feel the risk of IMing street ket is justified is really for you to decide, Hooter. As I said, I regularly IM krystal of known purity as do many others and I've never used a micron filter. Not saying it's a wise thing to do, but I do it often and am far from alone in that.
In the UK we just don't get vials cos we tend to get krystal direct from the labs before it's been made up into vials here. If you can get hold of krystal it should be 100% pure but powder is obviously far easier to kut and is not worth the risk, in my opinion. IVing ket is just a different experience that some enjoy and others don't. I like IM and IV for different reasons but holing via the IV method is fairly impractical, I'd say.