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 $27.78   4.314%   $28.9784   $28.98   $29.00   $29.00  
  Purchase #2 $131.55   5.712%   $139.0641   $139.06   $139.05   $139.10  
  Purchase #3 $74.28   9.597%   $81.4087   $81.41   $81.40   $81.40  
  Purchase #4 $175.72   6.834%   $187.7287   $187.73   $187.75   $187.70  
  Purchase #5 $112.37   9.607%   $123.1654   $123.17   $123.15   $123.20  
  Purchase #6 $25.29   5.737%   $26.7409   $26.74   $26.75   $26.70  
  Purchase #7 $174.86   13.588%   $198.62   $198.62   $198.60   $198.60  
  7 purchase totals = $721.85       $785.7062   $785.71   $785.70   $785.70