In most cases you can assume peak ac to be somewhat less then 2xVCT. So for a 2x18V transformer you get 2x18x2x2=144 (for single bridge). Meaning 200V diodes would be ok. That said go for higher voltage and current if you can afford it.
For a 0-80 or 40-0-40 transformer, yes that would be correct. 400 volt diodes would be a good idea. You can go lower but at the cost of more heat and shorter lifespan.