What is the best practice to store utf8 and us-ascii ?

[eric]

Hello,

In the MySQL Manual (10.7 Unicode Support) it's written:

<<Tip: To save space with UTF-8, use VARCHAR instead of CHAR. Otherwise, MySQL must reserve three bytes for each character in a CHAR CHARACTER SET utf8 column because that is the maximum possible length. For example, MySQL must reserve 30 bytes for a CHAR(10) CHARACTER SET utf8 column.
>>

I need utf8 to store data in several languages but I have also columns with characters only in us-ascii (for example an alphabetic code on one char or a bank account number with only numeric characters).
If the charset of the database is set in utf8, the columns with the type CHAR will spend too space if their contents is always in us-ascii.

What is the best practice in this case ?

Declare utf8 charset for the entire database and set a fixed charset (latin1) for the columns which contain only alphabetic characters? Or the reverse? Or an other solution?

Thanks for your opinion.
~~~
Eric