This may be a mechanic that's always existed with tanks but here's a couple lessons I learned the hard way.
1. Pumping out of tank, valve needs to be on bottom face or bottom row (not edge) of tank to get all the stuff out.
2. Destroying a valve destroys the contents of the tank, other blocks are safe. If you have two valves the contents are divided between the two valves (or more than two if you have multiple). Destroying a valve destroys the portion it was holding.
This is true, but if you don't put a valve in the bottom when you start out, you don't need to worry that much. I made this mistake with my first tank as well, but then I put a second valve in the bottom row, and was still able to get my lava out; so it was all good.
I tend to make the frames, roof, and base of my tanks out of stone brick, (I just think it looks very nice with the Steampunk texture pack, and it also has decent if not super blast resistance) and then make the central part of the tank walls with glass viewers, as I feel that they are usually cheap enough. The valve goes on the bottom row as mentioned, and I've also learned that it is generally a good idea to put an item I/O next to it sooner rather than later, as well, as it is always good to be able to grab a quick few cans of whatever you're storing at a moment's notice.
My standard size is also a 6x6x6 frame, which is 4x4x4 internally, and seems to allow for 1,024 buckets; I haven't needed anything bigger than that yet.
Like I said, this may be true of how all multi-block tanks work and since I'm new, I only found out because I've only done Xy's so far and no iron tanks yet.
I greatly prefer XyCraft tanks, because I consider spending that much iron on building a tank to be a poor allocation of resources. The ability to build a tank out of cheaper materials means that I can give myself some redundancy, rather than needing to build it as small as possible in order to conserve iron, which is something I never have enough of.
Also, if you're digging into a mountain or underground and make your tank the proper size but it doesn't work ... check the corners for sand/gravel/dirt ... I tried building one and it failed but eventually I found a chunk of dirt in a corner piece that I had missed.
I like lots (at least two blocks, but sometimes four) of clearance for my tanks, on all sides except the floor. This is partly because I generally want that much room for maneuverability as well, but it also makes sure that the tank doesn't conflict with nearby blocks as well.