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 $38.53   3.559%   $39.9013   $39.90   $39.90   $39.90  
  Purchase #2 $6.41   12.501%   $7.2113   $7.21   $7.20   $7.20  
  Purchase #3 $85.88   5.806%   $90.8662   $90.87   $90.85   $90.90  
  Purchase #4 $51.69   12.165%   $57.9781   $57.98   $58.00   $58.00  
  Purchase #5 $113.40   6.650%   $120.9411   $120.94   $120.95   $120.90  
  Purchase #6 $15.48   11.529%   $17.2647   $17.26   $17.25   $17.30  
  Purchase #7 $175.63   10.572%   $194.1976   $194.20   $194.20   $194.20  
  7 purchase totals = $487.02       $528.3603   $528.36   $528.35   $528.40