Heard someone mention that it is better to have multi grain multi fact requirements addressed at the report end, by creating a query for each fact and joining at the report level rather than dirctly do it in one query by draggin items from the packge. Is that true
That's true only when the package is not proper ! If the model is properly made with a proper DW behind the model then you can drag and drop items from multiple query subjects in the same query in report and Cognos will still create seperate queries and stitch them together. If you create multiple queries in the report you will be basically doing the same thing.
However when the model is made without a proper DW then the approach you mention is probably the way to go.