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 $19.25   3.803%   $19.9821   $19.98   $20.00   $20.00  
  Purchase #2 $160.64   5.210%   $169.0093   $169.01   $169.00   $169.00  
  Purchase #3 $115.44   12.102%   $129.4105   $129.41   $129.40   $129.40  
  Purchase #4 $55.90   11.476%   $62.3151   $62.32   $62.30   $62.30  
  Purchase #5 $90.50   10.394%   $99.9066   $99.91   $99.90   $99.90  
  Purchase #6 $222.67   4.451%   $232.581   $232.58   $232.60   $232.60  
  Purchase #7 $165.54   6.580%   $176.4325   $176.43   $176.45   $176.40  
  7 purchase totals = $829.94       $889.6371   $889.64   $889.65   $889.60