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 $21.88   3.670%   $22.683   $22.68   $22.70   $22.70  
  Purchase #2 $19.86   7.276%   $21.305   $21.31   $21.30   $21.30  
  Purchase #3 $121.46   9.598%   $133.1177   $133.12   $133.10   $133.10  
  Purchase #4 $191.81   7.489%   $206.1747   $206.17   $206.15   $206.20  
  Purchase #5 $87.42   5.274%   $92.0305   $92.03   $92.05   $92.00  
  Purchase #6 $75.81   13.134%   $85.7669   $85.77   $85.75   $85.80  
  Purchase #7 $9.27   5.287%   $9.7601   $9.76   $9.75   $9.80  
  7 purchase totals = $527.51       $570.8379   $570.84   $570.80   $570.90