OpenERP Developer Mode modification
No. That changes will not affect any xml file. Changes done directly from web-client will last until you update related module, those changes are not permanent. 
For permanent changes you have to change in xml file and update that module.
If we do some modification to our form views in Openerp 7 Developer mode (Ex : add onchange function and when it called some fields goes invisible)
is this modification automatically added to relevant view.xml files ?
I done such a thing.but my account_view.xml file not updated.but in database side ir_ui_view table saved with that record.please advice me to find that where is my missing point.?
EDITED : 
actually my account_view.xml cannot change.but when i'm change some changes to other view.xml files, they shows result when i restart the server & upgrade the modules.
but only when i change this account_view.xml file its not gives a result.but when i do some changes to account view through "Developer mode" they affected.
help me to seek what is the issue with that ?
(seems not using my account_view.xml) i tried to some simple changes also like menu item text change.they also not affected.but when i do for other module's view.xml they affected