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 $17.71   12.281%   $19.885   $19.89   $19.90   $19.90  
  Purchase #2 $7.67   6.801%   $8.1916   $8.19   $8.20   $8.20  
  Purchase #3 $104.67   9.373%   $114.4807   $114.48   $114.50   $114.50  
  Purchase #4 $108.69   10.485%   $120.0861   $120.09   $120.10   $120.10  
  Purchase #5 $137.58   3.675%   $142.6361   $142.64   $142.65   $142.60  
  Purchase #6 $37.55   6.475%   $39.9814   $39.98   $40.00   $40.00  
  Purchase #7 $100.81   12.128%   $113.0362   $113.04   $113.05   $113.00  
  7 purchase totals = $514.68       $558.2971   $558.31   $558.40   $558.30