AFAIK, you cannot change these defaults for new report items when you drag-drop or add them from the toolbox.
The product team made some design decisions regarding default values of certain elements, in order to keep the generated XML for the RDL a little leaner.
If you take a peak at the RDL with notepad, notice that when you have default report items without any customization, you should not actually find the defaults explicitly stated in the XML - they're baked into the code and the renderer (design time and runtime) implicitly applies them if not overridden by an explicit value.
However, if this is a critical deal for you, you "might could" write an XSLT transformation (or .NET app that parses XML with XPATH) for the RDL file, that finds the nodes you want and add the appropriate attributes and values and spits out the new RDL file to disk. Kinda risky...
Also, if you are using the Report Wizard, you can customize the layout styles and templates used by the wizard (like the cool blue one everybody likes to use). Here's an article with more details: http://beacspeak.spaces.live.com/blog/cns!69043F34B6D40843!125.entry
HTH
Thiago