Is it ok to use impersonation in the web.config file for a web service?

Let me tell you why I ask. My web service to ultimately connection to a sql
server database. The recommended way to connect to SqlServer is to use a
trusted connection (nt authentication etc), but inorder for me to do this I
want to change the user my webservice (aspnet_wp) is running under - don't
I? Hence the use of the Impersonation element in the web.config file. I
suppose I could use normal authentication to SqlServer but that means I have
to keep a username and password somewhere.

BYW, I'm using WSE 2.0 for authentication - don't know if this would make a
difference.

Thanks for any help

Graham Allwood