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 $58.91   12.391%   $66.2095   $66.21   $66.20   $66.20  
  Purchase #2 $61.58   10.516%   $68.0558   $68.06   $68.05   $68.10  
  Purchase #3 $71.89   11.647%   $80.263   $80.26   $80.25   $80.30  
  Purchase #4 $55.82   4.383%   $58.2666   $58.27   $58.25   $58.30  
  Purchase #5 $81.86   12.598%   $92.1727   $92.17   $92.15   $92.20  
  Purchase #6 $146.52   4.694%   $153.3976   $153.40   $153.40   $153.40  
  Purchase #7 $139.45   12.370%   $156.7   $156.70   $156.70   $156.70  
  7 purchase totals = $616.03       $675.0652   $675.07   $675.00   $675.20