Sorry, I really just meant the queuing for the basilica is what would cause me to drop it down the priority list. Other than that, it's a must-see. As bachslunch said, get there early (or late) and avoid the queue if you can. A lot of people get to the basilica early to 'avoid the queue' - and make a huge queue before it even opens. Perhaps a bit later, when that queue has gone in, there may be a time when the queue is at something like a minimum, but during peak season the queue always seems to stretch halfway down the Piazzetta. Find my tip on beating that queue (and people who tried it told me it worked for them), and go for it.

Regarding the Basilica, I did find this site yesterday:
http://www.alata.it/eng/BOOKING/sanmarco.asp

In theory at least, booking there is another method of avoiding the queue. I was able to book two spots on the 9:45AM opening slot Monday next week, the day after we arrive. The booking went smoothly, no charge, and I received a nice e-mail voucher. … Unlike the experience of trying to book a train ticket on the Trenitalia site!

As for getting lost in Venice, well, I suppose some people can get lost anywhere. But the city covers a relatively small area, it's bounded by water so you can't wander too far, and there are many signs directing you to the main sights.

For Rome, I think 5 days is definitely not too many (too few, if anything.) I would not cut into that time for a few days in Sorrento. Rome is much more enjoyable if you can explore it at a leisurely pace.
this thread gives ideas about the many wonderful things to see and do in Rome
http://www.fodors.com/community/euro...do-in-rome.cfm

As for traffic, I would go to great lengths to avoid driving in the city myself, but I didn't find traffic too disturbing as a pedestrian.