Safest way to uninstall the previous SQL Server versions
I strongly suggest to read Working with Multiple instance of SQL Server. If it is not interrupting at all STOP the SQL Server services which is not required its more safer approach. Microsoft has history of behaving incorrectly when instance is removed from server having multiple instances present. I have seen it multiple times 

IMHO If you have multiple instances its more safer to use command prompt to remove SQL Server instance as compared to GUI. Below is the dummy script. You have to open CMD as administrator and move to folder where Setp.exe file is present and then run below script.

Setup.exe /Action=Uninstall /FEATURES=SQL,AS,RS,IS,Tools /INSTANCENAME=MSSQLSERVER


You can read about uninstall parameters from Books Online. You can also have a look at how to uninstall existing instance of SQL Server

If you want to remove SQL Server 2005 This support article is very well written