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 $7.64   5.727%   $8.0775   $8.08   $8.10   $8.10  
  Purchase #2 $7.71   11.660%   $8.609   $8.61   $8.60   $8.60  
  Purchase #3 $141.97   11.812%   $158.7395   $158.74   $158.75   $158.70  
  Purchase #4 $193.88   8.515%   $210.3889   $210.39   $210.40   $210.40  
  Purchase #5 $102.50   10.848%   $113.6192   $113.62   $113.60   $113.60  
  Purchase #6 $83.34   5.437%   $87.8712   $87.87   $87.85   $87.90  
  Purchase #7 $70.80   12.236%   $79.4631   $79.46   $79.45   $79.50  
  7 purchase totals = $607.84       $666.7684   $666.77   $666.75   $666.80