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 $247.41   11.852%   $276.733   $276.73   $276.75   $276.70  
  Purchase #2 $64.27   7.261%   $68.9366   $68.94   $68.95   $68.90  
  Purchase #3 $94.74   12.428%   $106.5143   $106.51   $106.50   $106.50  
  Purchase #4 $132.59   8.796%   $144.2526   $144.25   $144.25   $144.30  
  Purchase #5 $222.81   11.219%   $247.8071   $247.81   $247.80   $247.80  
  Purchase #6 $43.29   11.280%   $48.1731   $48.17   $48.15   $48.20  
  Purchase #7 $79.83   11.708%   $89.1765   $89.18   $89.20   $89.20  
  7 purchase totals = $884.94       $981.5932   $981.59   $981.60   $981.60