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 $149.64   11.486%   $166.8277   $166.83   $166.85   $166.80  
  Purchase #2 $221.48   4.391%   $231.2052   $231.21   $231.20   $231.20  
  Purchase #3 $116.31   10.845%   $128.9238   $128.92   $128.90   $128.90  
  Purchase #4 $85.81   9.410%   $93.8847   $93.88   $93.90   $93.90  
  Purchase #5 $164.42   6.395%   $174.9347   $174.93   $174.95   $174.90  
  Purchase #6 $51.42   8.523%   $55.8025   $55.80   $55.80   $55.80  
  Purchase #7 $128.59   7.467%   $138.1918   $138.19   $138.20   $138.20  
  7 purchase totals = $917.67       $989.7704   $989.76   $989.80   $989.70