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 $12.71   6.714%   $13.5633   $13.56   $13.55   $13.60  
  Purchase #2 $6.70   11.212%   $7.4512   $7.45   $7.45   $7.50  
  Purchase #3 $138.83   11.344%   $154.5789   $154.58   $154.60   $154.60  
  Purchase #4 $144.95   3.414%   $149.8986   $149.90   $149.90   $149.90  
  Purchase #5 $34.45   4.833%   $36.115   $36.12   $36.10   $36.10  
  Purchase #6 $44.93   7.794%   $48.4318   $48.43   $48.45   $48.40  
  Purchase #7 $69.27   11.521%   $77.2506   $77.25   $77.25   $77.30  
  7 purchase totals = $451.84       $487.2894   $487.29   $487.30   $487.40