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 $8.80   4.241%   $9.1732   $9.17   $9.15   $9.20  
  Purchase #2 $114.62   13.705%   $130.3287   $130.33   $130.35   $130.30  
  Purchase #3 $214.25   7.139%   $229.5453   $229.55   $229.55   $229.50  
  Purchase #4 $73.41   6.440%   $78.1376   $78.14   $78.15   $78.10  
  Purchase #5 $204.37   8.319%   $221.3715   $221.37   $221.35   $221.40  
  Purchase #6 $174.85   9.314%   $191.1355   $191.14   $191.15   $191.10  
  Purchase #7 $26.67   13.653%   $30.3113   $30.31   $30.30   $30.30  
  7 purchase totals = $816.97       $890.0031   $890.01   $890.00   $889.90