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 $17.80   6.131%   $18.8913   $18.89   $18.90   $18.90  
  Purchase #2 $222.97   4.265%   $232.4797   $232.48   $232.50   $232.50  
  Purchase #3 $40.43   12.340%   $45.4191   $45.42   $45.40   $45.40  
  Purchase #4 $247.94   8.368%   $268.6876   $268.69   $268.70   $268.70  
  Purchase #5 $238.37   7.340%   $255.8664   $255.87   $255.85   $255.90  
  Purchase #6 $180.58   11.638%   $201.5959   $201.60   $201.60   $201.60  
  Purchase #7 $130.97   12.201%   $146.9496   $146.95   $146.95   $146.90  
  7 purchase totals = $1,079.06       $1,169.8896   $1,169.90   $1,169.90   $1,169.90