If you reset an IP phone that is currently registered and operational and the tftp server is not active. It would not matter since the earlier configuration has been saved on the phone and also the phone firmware had been copied over once.
You would only see problem if say the DHCP server is down and hence the phone would not be able to renew its ip address and that might have the phone stuck in configuring iP mode.
I just tested this and it shouldn't be an issue if
tftp is down for a phone reset.