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 $76.84   3.755%   $79.7253   $79.73   $79.75   $79.70  
  Purchase #2 $126.38   6.562%   $134.6731   $134.67   $134.65   $134.70  
  Purchase #3 $212.73   4.748%   $222.8304   $222.83   $222.85   $222.80  
  Purchase #4 $236.68   11.148%   $263.0651   $263.07   $263.05   $263.10  
  Purchase #5 $156.64   8.840%   $170.487   $170.49   $170.50   $170.50  
  Purchase #6 $89.96   4.171%   $93.7122   $93.71   $93.70   $93.70  
  Purchase #7 $45.49   3.357%   $47.0171   $47.02   $47.00   $47.00  
  7 purchase totals = $944.72       $1,011.5102   $1,011.52   $1,011.50   $1,011.50