I have a stupid habit of resetting my phone every month. Does this have any bad impact on WP7 phone??
There is a limit on the lifetime of flash memory used for storage within smartphones in the total number of times it can be rewritten.
Resetting the phone often will use up some of these writes, and reduce the overall lifetime by an amount, but I wouldn't expect it cause it to fail in the normal expected lifetime of the phone; after all, on low memory devices, the phone has the option to use memory paging which can have a dramatic effect on the number of reads/writes to flash memory.
I would be surprised if there was a good reason to wipe and start again so frequently - uninstalling is clean, and should suffice.
I have been reading a few pages (Nokia, LG, Samsung) on how to hard reset the device and haven't found any hint of this action causing some problems to the phone so I would say go for it. I am not really sure how it would do any harm because it's just "reloading" the original state into the phone.
The better question is why would you do something like that :)