Tips & Trick: Using single TNSNAMES.ORA on multiple Oracle Home
From REGEDIT, on every oracle home, under:
[HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE\HOMEx]
x = 0,1,2,3,...
create string key TNS_ADMIN and the value for TNS_ADMIN is directory path to your TNSNAMES.ORA.
eg. TNS_ADMIN="E:\ORACLE\ORA81\NETWORK\ADMIN"

Tips & Trick: Connecting to Oracle Server without using connect string
From REGEDIT, on every oracle home, under:
[HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE\HOMEx]
x = 0,1,2,3,...
create string key LOCAL and the value for LOCAL is connect string which you want use it for default connection.This connect string must be correctly configured in TNSNAMES.ORA.
eg. LOCAL="ORCL.WORLD"