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 $29.76   4.143%   $30.993   $30.99   $31.00   $31.00  
  Purchase #2 $111.57   9.514%   $122.1848   $122.18   $122.20   $122.20  
  Purchase #3 $84.48   8.867%   $91.9708   $91.97   $91.95   $92.00  
  Purchase #4 $44.61   12.165%   $50.0368   $50.04   $50.05   $50.00  
  Purchase #5 $73.57   7.743%   $79.2665   $79.27   $79.25   $79.30  
  Purchase #6 $41.42   3.201%   $42.7459   $42.75   $42.75   $42.70  
  Purchase #7 $116.27   7.264%   $124.7159   $124.72   $124.70   $124.70  
  7 purchase totals = $501.68       $541.9137   $541.92   $541.90   $541.90