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 $20.60   3.775%   $21.3777   $21.38   $21.40   $21.40  
  Purchase #2 $16.48   13.615%   $18.7238   $18.72   $18.70   $18.70  
  Purchase #3 $151.88   12.591%   $171.0032   $171.00   $171.00   $171.00  
  Purchase #4 $11.64   12.666%   $13.1143   $13.11   $13.10   $13.10  
  Purchase #5 $196.30   7.677%   $211.37   $211.37   $211.35   $211.40  
  Purchase #6 $111.97   5.317%   $117.9234   $117.92   $117.90   $117.90  
  Purchase #7 $51.26   8.539%   $55.6371   $55.64   $55.65   $55.60  
  7 purchase totals = $560.13       $609.1495   $609.14   $609.10   $609.10