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 $18.60   12.342%   $20.8956   $20.90   $20.90   $20.90  
  Purchase #2 $115.30   11.675%   $128.7613   $128.76   $128.75   $128.80  
  Purchase #3 $241.87   7.700%   $260.494   $260.49   $260.50   $260.50  
  Purchase #4 $212.66   5.206%   $223.7311   $223.73   $223.75   $223.70  
  Purchase #5 $108.86   5.278%   $114.6056   $114.61   $114.60   $114.60  
  Purchase #6 $104.42   3.571%   $108.1488   $108.15   $108.15   $108.10  
  Purchase #7 $219.74   8.509%   $238.4377   $238.44   $238.45   $238.40  
  7 purchase totals = $1,021.45       $1,095.0741   $1,095.08   $1,095.10   $1,095.00