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 $34.27   12.223%   $38.4588   $38.46   $38.45   $38.50  
  Purchase #2 $236.76   4.284%   $246.9028   $246.90   $246.90   $246.90  
  Purchase #3 $154.43   3.584%   $159.9648   $159.96   $159.95   $160.00  
  Purchase #4 $82.52   5.507%   $87.0644   $87.06   $87.05   $87.10  
  Purchase #5 $188.36   8.110%   $203.636   $203.64   $203.65   $203.60  
  Purchase #6 $47.72   9.329%   $52.1718   $52.17   $52.15   $52.20  
  Purchase #7 $90.26   3.379%   $93.3099   $93.31   $93.30   $93.30  
  7 purchase totals = $834.32       $881.5085   $881.50   $881.45   $881.60