I can't say for Soviet mission planning, but it would probably be similar to US planning. When flight profiles for nuclear missions were planned, information was available so that in addition to the usual route planning to avoid radar, AAA, and SAMs, you were able to plan around expected NUDETS (nuclear detonations) from missile strikes, other attacks etc so that you would not be in the area of a planned nuke going off thereby knocking you out of the sky. Avoiding downwind fallout, well you really could not do much dodging that because knowing the winds somewhere over enemy territory would be difficult. In any case, flying through a fallout cloud would not keep you from completing your mission. By the 1980s with luck most missions were not guaranteed to be one way - although tankers being when and where planned was going to be very iffy. Even then, especially for smaller aircraft with nuclear missions (such as all USN nuclear missions and some USAF), good luck meant exited out of the USSR before you had to land or bail out, not necessarily returning to a US base. I knew crews whose nuclear missions somewhat earlier ended with "after servicing the target head for (name of NATO or neutral country) and stretch your fuel. Bail out as late as possible then walk to the border." Not a terribly pleasant idea, bailing out over a country you've just dropped nukes on does not endear you to any locals you meet. I heard rumors, nothing I could confirm, that some crews who were going to do airbursts had decided to arm the weapons and then descend to the planned altitude without releasing the weapon - a quick and painless end, they mostly knew there was not going to be anyone to go back to.