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 $131.99   5.241%   $138.9076   $138.91   $138.90   $138.90  
  Purchase #2 $78.46   5.755%   $82.9754   $82.98   $83.00   $83.00  
  Purchase #3 $138.50   4.239%   $144.371   $144.37   $144.35   $144.40  
  Purchase #4 $145.30   6.504%   $154.7503   $154.75   $154.75   $154.80  
  Purchase #5 $97.85   7.518%   $105.2064   $105.21   $105.20   $105.20  
  Purchase #6 $107.56   12.601%   $121.1136   $121.11   $121.10   $121.10  
  Purchase #7 $79.57   13.288%   $90.1433   $90.14   $90.15   $90.10  
  7 purchase totals = $779.23       $837.4676   $837.47   $837.45   $837.50