Is it safe enough to password protect an important Excel document, or is it better to somehow provide additional protection for it?

If the document is Excel 2007-2013 (xlsx) and it doesn't contain data relating to national security, then there is no need for additional protection beyond a complex Password to Open it.

 The speed of a password search for xlsx files is relatively slow on even top-of-the-line GPUs. 

A complex password is a synthetic password that is made up of a random collection of at least eight different symbols (upper/lower case, digital, special symbols). That's it! If you lose one of these passwords, you aren't likely to recover it yourself!  :)

But if you set up a password something like "WorldCupBrazil2014", then it doesn't matter how strong a container you use, Excel, WinRar or TrueCrypt. A password like that can be broken at will in a few minutes.

IMPORTANT! Never use simple words as passwords if they are supposed to protect important information! Use password managers (keepass/1Password/ect.) to generate and store complex passwords.

However, if the document is Excel 97-2003, then things get much worse. These documents can be decrypted in a few minutes. There are special on-line services for that -, and it doesn't matter at all them how complex the password is. So, xls files must be archived prior to sending.

WinRar/7Zip/WinZip (provided AES encryption is included) provide reliable protection. And if you use a complex password, it goes without saying.

That's it in a nutshell.