Does holding down Ctrl/Shift while clicking refresh actually have an effect in modern browsers?

It does still work and helps bypass the cache - here's a break down of the shortcuts for the common browsers:

Internet Explorer

• Ctrl + F5
• Ctrl + click refresh button

Google Chrome

• Ctrl + F5
• Ctrl + click refresh button

Safari

• Version 4 or newer: Shift + click refresh button
• Version 3 or older: ⌘ + R

Firefox

• Ctrl + Shift + R
• Ctrl + F5 / ⌘ + F5
• Shift + click refresh button

Opera

• Ctrl + F5
• Shift + F5

Under Firefox 4, a normal refresh will generally reload basic content such as HTML as well as CSS and JavaScript. If you force a full refresh, the browser will reload all content, including images and all other objects, from the server.