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 $239.33   11.782%   $267.5279   $267.53   $267.55   $267.50  
  Purchase #2 $57.85   11.339%   $64.4096   $64.41   $64.40   $64.40  
  Purchase #3 $64.47   3.564%   $66.7677   $66.77   $66.75   $66.80  
  Purchase #4 $185.36   13.777%   $210.897   $210.90   $210.90   $210.90  
  Purchase #5 $52.57   9.830%   $57.7376   $57.74   $57.75   $57.70  
  Purchase #6 $170.84   9.765%   $187.5225   $187.52   $187.50   $187.50  
  Purchase #7 $62.25   10.586%   $68.8398   $68.84   $68.85   $68.80  
  7 purchase totals = $832.67       $923.7021   $923.71   $923.70   $923.60