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 $66.46   3.167%   $68.5648   $68.56   $68.55   $68.60  
  Purchase #2 $63.97   8.443%   $69.371   $69.37   $69.35   $69.40  
  Purchase #3 $148.96   11.840%   $166.5969   $166.60   $166.60   $166.60  
  Purchase #4 $91.74   6.678%   $97.8664   $97.87   $97.85   $97.90  
  Purchase #5 $180.25   12.462%   $202.7128   $202.71   $202.70   $202.70  
  Purchase #6 $134.75   3.132%   $138.9704   $138.97   $138.95   $139.00  
  Purchase #7 $224.61   5.432%   $236.8108   $236.81   $236.80   $236.80  
  7 purchase totals = $910.74       $980.8931   $980.89   $980.80   $981.00