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 $27.36   5.617%   $28.8968   $28.90   $28.90   $28.90  
  Purchase #2 $83.73   10.433%   $92.4656   $92.47   $92.45   $92.50  
  Purchase #3 $145.80   11.855%   $163.0846   $163.08   $163.10   $163.10  
  Purchase #4 $198.61   12.341%   $223.1205   $223.12   $223.10   $223.10  
  Purchase #5 $134.78   13.619%   $153.1357   $153.14   $153.15   $153.10  
  Purchase #6 $29.42   7.335%   $31.578   $31.58   $31.60   $31.60  
  Purchase #7 $87.32   12.109%   $97.8936   $97.89   $97.90   $97.90  
  7 purchase totals = $707.02       $790.1748   $790.18   $790.20   $790.20