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 $28.48   7.847%   $30.7148   $30.71   $30.70   $30.70  
  Purchase #2 $117.88   7.523%   $126.7481   $126.75   $126.75   $126.70  
  Purchase #3 $161.36   12.868%   $182.1238   $182.12   $182.10   $182.10  
  Purchase #4 $143.78   3.399%   $148.6671   $148.67   $148.65   $148.70  
  Purchase #5 $93.37   5.300%   $98.3186   $98.32   $98.30   $98.30  
  Purchase #6 $10.89   13.263%   $12.3343   $12.33   $12.35   $12.30  
  Purchase #7 $41.86   7.161%   $44.8576   $44.86   $44.85   $44.90  
  7 purchase totals = $597.62       $643.7643   $643.76   $643.70   $643.70