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 $50.59   13.452%   $57.3954   $57.40   $57.40   $57.40  
  Purchase #2 $152.73   5.573%   $161.2416   $161.24   $161.25   $161.20  
  Purchase #3 $209.88   11.169%   $233.3215   $233.32   $233.30   $233.30  
  Purchase #4 $203.27   13.488%   $230.6871   $230.69   $230.70   $230.70  
  Purchase #5 $232.94   11.272%   $259.197   $259.20   $259.20   $259.20  
  Purchase #6 $59.51   5.786%   $62.9532   $62.95   $62.95   $63.00  
  Purchase #7 $77.57   6.605%   $82.6935   $82.69   $82.70   $82.70  
  7 purchase totals = $986.49       $1,087.4893   $1,087.49   $1,087.50   $1,087.50