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 $159.58   12.791%   $179.9919   $179.99   $180.00   $180.00  
  Purchase #2 $64.76   8.599%   $70.3287   $70.33   $70.35   $70.30  
  Purchase #3 $9.94   6.165%   $10.5528   $10.55   $10.55   $10.60  
  Purchase #4 $38.75   3.663%   $40.1694   $40.17   $40.15   $40.20  
  Purchase #5 $29.51   4.512%   $30.8415   $30.84   $30.85   $30.80  
  Purchase #6 $247.62   13.197%   $280.2984   $280.30   $280.30   $280.30  
  Purchase #7 $129.54   11.465%   $144.3918   $144.39   $144.40   $144.40  
  7 purchase totals = $679.70       $756.5745   $756.57   $756.60   $756.60