as a draughtsman flat is best, rolled loosly is second best, if you have a 4" tube roll them round the outside, a bigger tube would be better, you can make a drawing storage system using 12" sections of drainage pipe (4") and just stand them on end and glue / fix them all together. (I used the 3" cardboard cores from A3 rolls of printer paper to make mine, but that leaves the plans a bit too tightly rolled).
remember all paper drawings will stretch and shrink up to about 2% when damp or dry, as they will also do in the printing process, so always check against the scale bar on the drawing, unfortunately the paper can stretch unequally depending upon the grain direction (this is why we used to draw on film) an A0 drawing (1281mm long) can stretch by 3mm when damp.
Just remember its worth checking the scale. I have had many people over the years complaining that their measurements they scaled off the drawing were wrong, and this is generally the cause and why no plan printer manufacturer will ever guarantee a higher accuracy than about 3%.
Grendel