There is no one size fits all answer to this, as is so often the case.
Generally though, if you can precompute it, do it.
If your robot doesn't change all the time, so for example you build it and it stays the same the whole time, then what's the point of calculating all of its parts every frame. But if it does change all the time then obviously you can't get away with making all of it one big object.
You are probably aware that there are tons of things that become computationally more expensive the more objects you have, for example collision detection. So you have an interest to keep the number of game objects low(-ish).
When it comes to rendering, ideally you would want to have a VBO and IBO for your whole robot, so you can push it over to the GPU once and then just reference it forever. If instead you split your robot up into bits you will have to make a draw call for each of the bits. While the amount of drawing that your GPU ends up doing will be the same, the amount of work for your CPU to translate all of those instructions for the GPU will be much higher.
So in most cases treating it as one big object would be better. However there are cases where the other treatment might work better as well.
Computing everything you need to treat your robot as one object is relatively computationally intensive and memory heavy. (Compared to just keeping everything as a collection of blocks) So if you are in a garbage collected environment and want to minimize your memory footprint, if each time a block is changed you have to recalculate the big object it could cause a garbage collection every time and make your game lag for a frame.