questions for owner of 20" w/summer tire?

[us_matrix]

I wonder you have to change winter tire or all-sesaons tire when you drive in snow if you have 20" w/summer tire. Here is my quesiton. If I am going to upgrade from 19" (255/50) to 20" staggered setup, I think 4 wheel alignment must be needed after I upgraded to 20"....
If that is the case, what happen if I switch back to my 19" all-season for driving in snow just for short period of time (like a week or two to Lake Tahoe ski) and then switch it back to 20" staggered setup? Do I have to do alignment again? Would that screw up the alignment setting since I am changing back and forth in different size 19" 255/50 to 20" 315/40 ? What do you guys do? Do you do alignment again after you switch back 20" rims and tire?

Thanks.