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 $58.63   12.872%   $66.1769   $66.18   $66.20   $66.20  
  Purchase #2 $232.59   10.498%   $257.0073   $257.01   $257.00   $257.00  
  Purchase #3 $137.48   12.526%   $154.7007   $154.70   $154.70   $154.70  
  Purchase #4 $27.68   10.356%   $30.5465   $30.55   $30.55   $30.50  
  Purchase #5 $198.58   12.521%   $223.4442   $223.44   $223.45   $223.40  
  Purchase #6 $138.34   6.119%   $146.805   $146.81   $146.80   $146.80  
  Purchase #7 $205.47   10.385%   $226.8081   $226.81   $226.80   $226.80  
  7 purchase totals = $998.77       $1,105.4887   $1,105.50   $1,105.50   $1,105.40