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 $5.89   12.499%   $6.6262   $6.63   $6.65   $6.60  
  Purchase #2 $227.58   7.517%   $244.6872   $244.69   $244.70   $244.70  
  Purchase #3 $166.84   8.227%   $180.5659   $180.57   $180.55   $180.60  
  Purchase #4 $58.85   13.347%   $66.7047   $66.70   $66.70   $66.70  
  Purchase #5 $181.79   13.544%   $206.4116   $206.41   $206.40   $206.40  
  Purchase #6 $84.41   6.190%   $89.635   $89.64   $89.65   $89.60  
  Purchase #7 $246.91   7.742%   $266.0258   $266.03   $266.05   $266.00  
  7 purchase totals = $972.27       $1,060.6564   $1,060.67   $1,060.70   $1,060.60