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 $11.58   3.512%   $11.9867   $11.99   $12.00   $12.00  
  Purchase #2 $45.71   9.673%   $50.1315   $50.13   $50.15   $50.10  
  Purchase #3 $36.49   11.319%   $40.6203   $40.62   $40.60   $40.60  
  Purchase #4 $135.82   10.233%   $149.7185   $149.72   $149.70   $149.70  
  Purchase #5 $108.68   5.573%   $114.7367   $114.74   $114.75   $114.70  
  Purchase #6 $171.48   11.367%   $190.9721   $190.97   $190.95   $191.00  
  Purchase #7 $81.93   6.697%   $87.4169   $87.42   $87.40   $87.40  
  7 purchase totals = $591.69       $645.5827   $645.59   $645.55   $645.50