An excellent question, and I think the answer comes down to personal preference. When fishing (or planning to fish) dropper rigs (one fly tied off the hook bend or the tag end of the leader) I always tie them up in advance and keep them in a dropper rig flybox (available from Orvis and others). On the other hand for fishing individual flies I usually tie them on, on stream.
That being said however there is absolutely nothing wrong with having a small piece of appropriately sized tippet pre-tied to the fly so you can switch flies on stream. It will certainly ensure you are using up less of your tippet with every fly change.
As to the necessary tools, well that is very much again personal preference. What I consider necessary you may not. I think a pair of hemostats, a pair of nippers (can be fingernail clippers from the drug store), a few spools of appropriately sized tippet, a bottle of flotant (if fishing dry flies), sunscreen, and of course a fly box or two. I tend to be one of those "kitchen sink" people so I have multiple (seven) flyboxes, multiple types of flotant (gell and powder), enough tippet to completely rebuild a leader if necessary, etc. But there are also times I go fishing and all I have is a flybox and my rod/reel.
Hope this helps and welcome to the best fly fishing site on the net.
Brian
"My father was very sure about certain matters pertaining to the universe. To him, all good things - trout as well as eternal salvation - come by grace and grace comes by art and art does not come easy." Norman Maclean, A River Runs Through It