Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $170.86   11.136%   $189.887   $189.89   $189.90   $189.90  
  Purchase #2 $138.69   4.382%   $144.7674   $144.77   $144.75   $144.80  
  Purchase #3 $22.53   13.564%   $25.586   $25.59   $25.60   $25.60  
  Purchase #4 $133.72   8.641%   $145.2747   $145.27   $145.25   $145.30  
  Purchase #5 $197.74   13.225%   $223.8911   $223.89   $223.90   $223.90  
  Purchase #6 $184.58   4.674%   $193.2073   $193.21   $193.20   $193.20  
  Purchase #7 $63.85   6.854%   $68.2263   $68.23   $68.25   $68.20  
  7 purchase totals = $911.97       $990.8398   $990.85   $990.85   $990.90