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 $227.77   13.280%   $258.0179   $258.02   $258.00   $258.00  
  Purchase #2 $54.42   3.698%   $56.4325   $56.43   $56.45   $56.40  
  Purchase #3 $207.78   7.231%   $222.8046   $222.80   $222.80   $222.80  
  Purchase #4 $226.51   3.860%   $235.2533   $235.25   $235.25   $235.30  
  Purchase #5 $145.84   5.314%   $153.5899   $153.59   $153.60   $153.60  
  Purchase #6 $129.53   3.472%   $134.0273   $134.03   $134.05   $134.00  
  Purchase #7 $58.99   9.494%   $64.5905   $64.59   $64.60   $64.60  
  7 purchase totals = $1,050.84       $1,124.7160   $1,124.71   $1,124.75   $1,124.70