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 $240.79   11.683%   $268.9215   $268.92   $268.90   $268.90  
  Purchase #2 $30.39   6.443%   $32.348   $32.35   $32.35   $32.30  
  Purchase #3 $249.31   9.713%   $273.5255   $273.53   $273.55   $273.50  
  Purchase #4 $23.59   7.562%   $25.3739   $25.37   $25.35   $25.40  
  Purchase #5 $243.81   10.228%   $268.7469   $268.75   $268.75   $268.70  
  Purchase #6 $109.51   3.114%   $112.9201   $112.92   $112.90   $112.90  
  Purchase #7 $73.36   3.612%   $76.0098   $76.01   $76.00   $76.00  
  7 purchase totals = $970.76       $1,057.8457   $1,057.85   $1,057.80   $1,057.70