Right side up is fine
I did it right side up and it has worked fine thus far. Just yesterday I rolled the fuselage to its side to rivet the main seam and floor skins. If I ever had the time, I'd rewrite the sequence of some stuff; there's a lot you can do before you hang the firewall on re: seats, fuel lines, etc. Much of that is/would be easier to fit before rather than after.
Careful, though. Read ahead, think about sequence, and plan your tasks -- there are plenty of gotcha's lurking about. It's flattering that the plans get less detailed once you're this far along, but in some places the sequence is simply not really well thought out. Of course it is very satisfying to build successfully without having your hand held.
Something that makes it difficult is that there is widespread lack of consistency. In some areas you get lots of detail and explanation, in others very little. Some drawings appear to have very tight tolerances for parts whose dimensions would have been fine if they rounded, while some that seem to matter down to a 1/32 aren't noted as such. There is, to me, a feeling that many hands worked on the plans over long periods of time and that there hasn't been a comprehensive look at them with an eye towards optimizing both the sequence and the level of detail.
As always, it is YOUR build and YMMV
George