I think top down and bottom up are one of those things invented by academics. In reality people use a combination of both strategies, making some parts in isolation and some in the context of other parts.
In CATIA you have the freedom to do whatever best suits your requirements. You can build all your parts in isolation and then assemble them. Or you can build parts in an assembly where you just use the other parts as scenery and build around them. Or you can build parts in an assembly where you import isolated pieces of geometry from the other part in the assembly and don't maintain any links, relying on the replace functions to update when required. Or you can work fully contexturally where geometry is imported and maintains associativity with the source geometry. Or you can mix an match.
For example I might start bottom up and make the basic form of a couple of major components, prehaps a skelton. Place them in an assembly so I have some context for the mating parts. I then build a number of other components in my assembly but don't have any associativity. I might then add a sub product, include the skeleton again and build some associative parts but limit the context to the sub product (in context associativity works well where the design intent is clear, I use it for hoses (a hose goes from a->b and never does anything else))
To work in context in a product, just double click on the part object in the tree. link creation is determined by the options you have set (tools->options->infrastructure->Part infrastructure). I also strongly recommend understanding and using publications before creating links. I also recommend you find out about the skeleton method as this is an excelent way of structuring your model and managing the links between objects (avoid the spiders web of links it will cause you pain and be slow. Be very careful adding constraint to contexturally driven parts - you cannot position something where its position is defined contexturally)
