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 $8.37   8.261%   $9.0614   $9.06   $9.05   $9.10  
  Purchase #2 $9.49   12.399%   $10.6667   $10.67   $10.65   $10.70  
  Purchase #3 $18.93   12.811%   $21.3551   $21.36   $21.35   $21.40  
  Purchase #4 $40.99   3.430%   $42.396   $42.40   $42.40   $42.40  
  Purchase #5 $166.60   10.830%   $184.6428   $184.64   $184.65   $184.60  
  Purchase #6 $235.48   10.147%   $259.3742   $259.37   $259.35   $259.40  
  Purchase #7 $14.56   11.763%   $16.2727   $16.27   $16.25   $16.30  
  7 purchase totals = $494.42       $543.7689   $543.77   $543.70   $543.90