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 $181.28   4.285%   $189.0478   $189.05   $189.05   $189.00  
  Purchase #2 $147.95   6.620%   $157.7443   $157.74   $157.75   $157.70  
  Purchase #3 $17.39   10.794%   $19.2671   $19.27   $19.25   $19.30  
  Purchase #4 $73.89   11.406%   $82.3179   $82.32   $82.30   $82.30  
  Purchase #5 $240.51   7.820%   $259.3179   $259.32   $259.30   $259.30  
  Purchase #6 $121.97   6.349%   $129.7139   $129.71   $129.70   $129.70  
  Purchase #7 $146.77   3.606%   $152.0625   $152.06   $152.05   $152.10  
  7 purchase totals = $929.76       $989.4714   $989.47   $989.40   $989.40