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 $13.98   8.518%   $15.1708   $15.17   $15.15   $15.20  
  Purchase #2 $18.62   5.650%   $19.672   $19.67   $19.65   $19.70  
  Purchase #3 $58.51   4.247%   $60.9949   $60.99   $61.00   $61.00  
  Purchase #4 $10.73   3.168%   $11.0699   $11.07   $11.05   $11.10  
  Purchase #5 $20.27   10.119%   $22.3211   $22.32   $22.30   $22.30  
  Purchase #6 $211.55   12.678%   $238.3703   $238.37   $238.35   $238.40  
  Purchase #7 $83.50   11.272%   $92.9121   $92.91   $92.90   $92.90  
  7 purchase totals = $417.16       $460.5111   $460.50   $460.40   $460.60