cfqueryparam with PreserveSingleQuotes

> Is the following query safe against SQL injection attacks?

Yes. It is cfqueryparam that makes it safe. However, your query should be something like
<cfquery name="AddRequestParams" datasource="MyDb">
INSERT INTO MyTable (Field)
VALUES (<cfqueryparam cfsqltype="cf_sql_varchar" value = "#URL.Value#">)
</cfquery>

> I've read in several places that you should avoid
> using PreserveSingleQuotes with cfqueryparam

That is true.

> ... seen no examples of why this would be dangerous.

I am not aware of any such danger either. The reason for not using preserveSingleQuotes with cfqueryparam is because the cfqueryparam tag automatically preserves single quotes.

preservesinglequotes serves no useful purpose in your example, with or without cfqueryparam.

preservesinglequotes is used like this:

sql = "select somefields from sometables where somefield = '#somevariable#' ;

<cfquery>
#preservesinglequotes(sql)#
</cfquery>