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 $5.71   11.835%   $6.3858   $6.39   $6.40   $6.40  
  Purchase #2 $133.83   9.781%   $146.9199   $146.92   $146.90   $146.90  
  Purchase #3 $241.57   13.391%   $273.9186   $273.92   $273.90   $273.90  
  Purchase #4 $216.91   3.343%   $224.1613   $224.16   $224.15   $224.20  
  Purchase #5 $102.32   8.702%   $111.2239   $111.22   $111.20   $111.20  
  Purchase #6 $63.54   11.347%   $70.7499   $70.75   $70.75   $70.70  
  Purchase #7 $141.54   4.540%   $147.9659   $147.97   $147.95   $148.00  
  7 purchase totals = $905.42       $981.3253   $981.33   $981.25   $981.30