Where to put custom function at?

Let's say I make a custom function, do I make it a CFC or a tag?

Here is some sample code (I put in bold my function call)

<cfif IsDefined("form.submit")>

    <cfset data.comment = Trim(HTMLEditFormat(form.comment))>
    <cfset data.name = Trim(REReplace(form.name, "[^A-Za-z0-9 ]", "", "all"))>
    <cfset data.location = Trim(CustomFunction(form.location))><!--- where to put it? as cfc or as a tag? --->
    <cfif data.comment NEQ "">       
        <cfif data.name EQ "">
            <cfset data.name = "Anonymous">
        </cfif>
        <cfinvoke
        component="guestbook.cfc.db"
        method="insert_comment">
        <cfinvokeargument name="form_data" value="#data#"/>
        </cfinvoke>
    <cfelse>
        <cfset message="Comment is required!">
    </cfif>
</cfif>