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 $15.35   9.540%   $16.8144   $16.81   $16.80   $16.80  
  Purchase #2 $11.64   4.586%   $12.1738   $12.17   $12.15   $12.20  
  Purchase #3 $78.27   13.387%   $88.748   $88.75   $88.75   $88.70  
  Purchase #4 $10.75   3.618%   $11.1389   $11.14   $11.15   $11.10  
  Purchase #5 $145.50   8.588%   $157.9955   $158.00   $158.00   $158.00  
  Purchase #6 $78.61   12.167%   $88.1745   $88.17   $88.15   $88.20  
  Purchase #7 $46.90   4.600%   $49.0574   $49.06   $49.05   $49.10  
  7 purchase totals = $387.02       $424.1025   $424.10   $424.05   $424.10