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 $145.57   8.172%   $157.466   $157.47   $157.45   $157.50  
  Purchase #2 $154.28   3.285%   $159.3481   $159.35   $159.35   $159.30  
  Purchase #3 $140.75   12.693%   $158.6154   $158.62   $158.60   $158.60  
  Purchase #4 $20.59   11.759%   $23.0112   $23.01   $23.00   $23.00  
  Purchase #5 $23.64   7.854%   $25.4967   $25.50   $25.50   $25.50  
  Purchase #6 $17.66   6.679%   $18.8395   $18.84   $18.85   $18.80  
  Purchase #7 $117.68   5.316%   $123.9359   $123.94   $123.95   $123.90  
  7 purchase totals = $620.17       $666.7128   $666.73   $666.70   $666.60