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 $129.49   9.494%   $141.7838   $141.78   $141.80   $141.80  
  Purchase #2 $53.29   4.838%   $55.8682   $55.87   $55.85   $55.90  
  Purchase #3 $50.85   5.136%   $53.4617   $53.46   $53.45   $53.50  
  Purchase #4 $143.93   9.447%   $157.5271   $157.53   $157.55   $157.50  
  Purchase #5 $50.93   9.166%   $55.5982   $55.60   $55.60   $55.60  
  Purchase #6 $228.72   7.805%   $246.5716   $246.57   $246.55   $246.60  
  Purchase #7 $209.57   8.849%   $228.1148   $228.11   $228.10   $228.10  
  7 purchase totals = $866.78       $938.9254   $938.92   $938.90   $939.00