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 $27.74   4.312%   $28.9361   $28.94   $28.95   $28.90  
  Purchase #2 $197.59   3.353%   $204.2152   $204.22   $204.20   $204.20  
  Purchase #3 $108.71   12.729%   $122.5477   $122.55   $122.55   $122.50  
  Purchase #4 $128.99   9.700%   $141.502   $141.50   $141.50   $141.50  
  Purchase #5 $74.47   4.337%   $77.6998   $77.70   $77.70   $77.70  
  Purchase #6 $16.79   12.696%   $18.9217   $18.92   $18.90   $18.90  
  Purchase #7 $40.58   10.340%   $44.776   $44.78   $44.80   $44.80  
  7 purchase totals = $594.87       $638.5985   $638.61   $638.60   $638.50