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 $121.67   7.746%   $131.0946   $131.09   $131.10   $131.10  
  Purchase #2 $149.27   6.817%   $159.4457   $159.45   $159.45   $159.40  
  Purchase #3 $24.79   10.461%   $27.3833   $27.38   $27.40   $27.40  
  Purchase #4 $108.79   10.283%   $119.9769   $119.98   $120.00   $120.00  
  Purchase #5 $8.90   11.131%   $9.8907   $9.89   $9.90   $9.90  
  Purchase #6 $15.42   6.480%   $16.4192   $16.42   $16.40   $16.40  
  Purchase #7 $101.83   13.629%   $115.7084   $115.71   $115.70   $115.70  
  7 purchase totals = $530.67       $579.9188   $579.92   $579.95   $579.90