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 $12.39   3.646%   $12.8417   $12.84   $12.85   $12.80  
  Purchase #2 $19.45   10.317%   $21.4567   $21.46   $21.45   $21.50  
  Purchase #3 $221.61   4.564%   $231.7243   $231.72   $231.70   $231.70  
  Purchase #4 $136.46   8.601%   $148.1969   $148.20   $148.20   $148.20  
  Purchase #5 $58.59   12.755%   $66.0632   $66.06   $66.05   $66.10  
  Purchase #6 $34.93   7.486%   $37.5449   $37.54   $37.55   $37.50  
  Purchase #7 $144.93   6.353%   $154.1374   $154.14   $154.15   $154.10  
  7 purchase totals = $628.36       $671.9651   $671.96   $671.95   $671.90