Depends on the situation. In league, lineman go on the line early on. In tournament, I like blitzers up front because they're more durable.
In league, I would put blitzers on only early on when my linewomen didn't have skills, or later on when facing a Elflike team without any exceptional strength and guard. A few guard Blitzers can certainly make a big difference then.
If you are fortunate enough to get one stand firm/guard blitzer, put her right in the middle of the front three. If you're amazingly lucky enough to have to of them, put them on the inside of the formation - it makes it much more difficult for your oponent to get around them.