I’m not particularly sure why you’d want to set “reply all” as the default response to emails, but apparently, there are a number of people who do. Personally, unless absolutely necessary, I find reply all responses very annoying and far too cumbersome on the inbox, not to mention the damage potential.
Setting your default reply all is a matter of clicking a couple of general settings in Gmail.
To access the account settings, click the menu button, then click Settings.
You’ll next see your various Google accounts.
Click on General settings to pull up the universal account options. Click the box next to Reply all to set “reply all” as the default message reply method.
Now you can exit out of the settings and your responses to messages will automatically include all recipients. Just remember though, it’s important to make sure that you’re not compromising your client’s confidentiality by exposing the whole world.
My preference, if you must reply all, is to use the message menu > Reply all option.