If my employer is in Jackson, MI and I live in Grand Rapids, MI. But, I travel to a different city checking the job sites. What do I put for actual work location?

It depends on why you are asking. If you are trying to figure out if you need to file local taxes for the various job sites, you would need to contact the local taxing authority as different jurisdictions have different rules. 

For your state and federal taxes you would use Grand Rapids since that is your actual primary work location.