Two quick questions regarding restriction in XML Schema

I think these are related.. Can someone tell me --

a) If we derive types by restriction, do we have to use xsi:type in the
document? Is it true if I say that the document will be valid irrespective
of whether we use xsi:type or not .. We use xsi:type to know the "precise"
type??

b) Is there any point in using block for restriction

thanks and regards - murali.