A decreasing Tax Rate Percentage is generally a positive sign, but you will need to investigate why this is occurring. Some possible reasons could be the company has moved some of its operations to an area with a different Tax Rate, or the company is able to record a different value for the Income Tax than the income tax it actually had to pay.
The Tax Rate imposed on a company does not usually change much, and any change needs to be researched further as this can be an indicator of large changes occurring within a company.