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 $211.43   6.208%   $224.5556   $224.56   $224.55   $224.60  
  Purchase #2 $55.82   5.663%   $58.9811   $58.98   $59.00   $59.00  
  Purchase #3 $177.52   4.428%   $185.3806   $185.38   $185.40   $185.40  
  Purchase #4 $14.39   7.510%   $15.4707   $15.47   $15.45   $15.50  
  Purchase #5 $200.70   8.583%   $217.9261   $217.93   $217.95   $217.90  
  Purchase #6 $243.60   5.857%   $257.8677   $257.87   $257.85   $257.90  
  Purchase #7 $89.66   7.631%   $96.502   $96.50   $96.50   $96.50  
  7 purchase totals = $993.12       $1,056.6838   $1,056.69   $1,056.70   $1,056.80