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I am building a kitchen base unit (frameless type) out of melamine plywood. Regarding the assembly of the "box" is it OK for the side panel to rest on top of the bottom panel, where the screws will go up from the bottom? This means that the edge of the bottom panel will be visible, though that is not an issue for me.
In most, if not all, of the pictures and videos I have seen so far the side panel goes adjacent to the bottom panel, and the screws goes in from the side.
As LosManos said, a picture can be very helpful in these kinds of situations. However, I think I have an idea what you mean.
It really has to do with what kind of strength the board need to provide to the case. Melemine does not have a lot of strength to hold a screw from pulling straight out. So if zipping in screws to the bottom is what is mostly going to be holding the top and bottom together, it will be fairly weak. If you don't plan to move it, such as permanent kitchen cabinets, then this isn't such a big deal.
Otherwise, I would make a stronger frame on the inside where you can put shorter screws through a cross member (without going all the way through the melamine) This would be a stronger case and less stresses on the screws in the weak material, the shearing forces would be better than direct pulling.
Setting the sides on the base is fine -- good, even -- as long as you have structural support between the floor and the base of the cabinet. That way, the load on the cabinet is being carried by wood, not screws.
The way you've seen it elsewhere is generally because the sides go all the way to the floor (thus carrying the load directly through the sheet goods).
And your comment about the bare edge might be solved with "edge banding tape".
There are a couple reasons why it's preferable to have the sides extend all the way to the bottom, rather than having them rest on the bottom. First, it looks nicer--you have one continuous piece all the way down the side. Second, it provides a little more structural support against racking. Whether you're using screws or nails and glue, attaching to the bottom across multiple axes (vertical for middle partitions, horizontal for the ends) adds strength.
That said, if you simply attach even a partial back to your cabinet, that will add all the racking resistance you need. Adding a face frame on the front will help, too.
You mentioned that you're using screws, and it sounds like you're mostly using butt joints for construction. If you don't want to be able to see the screws on the outside of the cabinet, you should consider using pocket screws and/or gluing and screwing solid wood cleats to all the plywood pieces. Screws run through the edge of a piece of plywood can split the plywood, but screws through the face hold better. Normal wood glue doesn't stick very well to melamine, so if the plywood is coated on both sides, you may need to use polyurethane glue (Gorilla Glue's namesake adhesive).
I would say it's not a modern method to have any wood going down to the floor. Good quality cabinets sit on plastic legs keeping all wood above any leakage from washing machines, dish washers, fridges etc. Just looked at mine and surprisingly the sides do not sit on top of the bottom panel, but screw to the side. They do have very good fixing system not just screws, they support a granite work top so must be strong enough.
