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 $61.51   8.192%   $66.5489   $66.55   $66.55   $66.50  
  Purchase #2 $124.60   7.844%   $134.3736   $134.37   $134.35   $134.40  
  Purchase #3 $129.58   3.480%   $134.0894   $134.09   $134.10   $134.10  
  Purchase #4 $50.39   12.450%   $56.6636   $56.66   $56.65   $56.70  
  Purchase #5 $24.78   8.512%   $26.8893   $26.89   $26.90   $26.90  
  Purchase #6 $37.28   11.544%   $41.5836   $41.58   $41.60   $41.60  
  Purchase #7 $71.38   4.761%   $74.7784   $74.78   $74.80   $74.80  
  7 purchase totals = $499.52       $534.9268   $534.92   $534.95   $535.00