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 $122.88   5.649%   $129.8215   $129.82   $129.80   $129.80  
  Purchase #2 $91.25   6.307%   $97.0051   $97.01   $97.00   $97.00  
  Purchase #3 $137.69   12.413%   $154.7815   $154.78   $154.80   $154.80  
  Purchase #4 $72.31   7.208%   $77.5221   $77.52   $77.50   $77.50  
  Purchase #5 $168.36   8.211%   $182.184   $182.18   $182.20   $182.20  
  Purchase #6 $127.66   11.396%   $142.2081   $142.21   $142.20   $142.20  
  Purchase #7 $171.67   3.740%   $178.0905   $178.09   $178.10   $178.10  
  7 purchase totals = $891.82       $961.6128   $961.61   $961.60   $961.60