Answer is "it depends".
DBAs use Toad all the time on their production databases in order to perform change management, alter objects as necessary, diagnose issues, track sessions, etc.
Pretty much any product that connects to Oracle (even command line, like SQL*Plus) will be creating *some* amount of overhead on the database. Bottom line is that certain types of activity are more expensive than others. Example: running a 2-hour-long script within Toad's editor to refresh analyze stats on all tables, or reorganizing tablespaces, etc. will put more overhead on the database than, say, having Toad reverse-engineer the DDL for a few objects.
Hope this helps.