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can some one tell it is ok to modify the magento pages(like header.phtml, or some controller) modify means adding or deleting some code from magento pages.
simple create my new pages for that modification
about php files its better to keep them in local code pool,
about templates - i am doing in this way: on local pc i have 2 themes, 1st is default, 2nd with only changed templates from 1st theme,
on production server i merge my changes from 2nd theme to default one