i've had pretty good success using older glass with just slight distortion and having it slivered. The guy I take it to for silvering puts the silver on thinner than normal so there's a slight translucency in areas of the miirror. I also had him not paint the back on the first batch as Dennis suggested to allow it to tarnish but I allowed it to go too long and the mirror became too dark, almost like a smoked glass mirror- so it had to be stripped and redone. Now, just the thinned silvering and then painted on back.
One thing I found about old glass- if it is really old and has a lot of distortion, don't use it for a mirror, it will look like carnival mirror with so much distortion. I prefer glass from the 30's -40's with only slight waviness that will give you slight- but noticeable distortion. Hope this helps.