Most of the tin work you are going to do will be 20g (.0375) or 18g (.050) so I usually keep a few 2x4 sheets of that around. For the plasma art I use 16g(.0625), then I keep some 1/8"(.125) and 1/4"(.250) Rarely will you ever need larger sheets than that unless you are doing something specific, 90% of my sheets gets cut down into smaller pieces. Plus it may not sound like much but working even a 2x4 sheet is about what a single guy can do easily... much more than that and its a two man job not buggering it up as you try to work it. I've got a 52" shear but since I work by myself it is easier just to pay the couple $ cut fee and have it knocked down to 2x4.
Like the guys said... have a general stock of odds and ends. You will always find use for 1"x1" and 3/4"x 3/4" angle, I've got flat stock in all types of widths from 1/8" to 1/2" thickness. I use DOM tubing for making spacers and standoffs so I keep that on hand in 1/4", 5/16", 3/8" and 1/2" ID in steel and aluminum. Now that I've got the mill and lathe in house I've got more solid stock that I can machine what I need... I scored a 4' section of 3" round aluminum for about $50 in the drop area one day which is great for cutting belt tensioner standoffs and pulley standoffs, hubcentric rings.... and all kinds of other fun!! Again... it sounds like a lot but I usually just pick up a 1-3' piece when I see it in the drop section... My metal yard has a HUGE well organized drop section so it makes it easy.
I usually keep a plate of 14g aluminum for making tanks and some thicker aluminum up to 5/8" for machining brackets. Some flat expanded metal for making oil cooler shields...
The more your in house capabilities the more use you will find for odds and ends. I've built 2x4 shelves under my bench for the sheet stock and am going to make some stand up "files" on the side to keep my drops organized by size. Best way I've found to organize the sticks is go to your hardware store and buy a couple 10' sections of 4" pvc.. cut them into 3 even sections and cap the end on the inside with a 3" pvc cap.... glue them together and instant organizer. Big enough to get your hand down inside for short pieces, tall enough to stand up 6-8' pieces with ease.
....looks like it's time to reorganize the organizer....