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 $9.45   4.767%   $9.9005   $9.90   $9.90   $9.90  
  Purchase #2 $154.29   9.359%   $168.73   $168.73   $168.75   $168.70  
  Purchase #3 $245.98   8.824%   $267.6853   $267.69   $267.70   $267.70  
  Purchase #4 $242.29   11.307%   $269.6857   $269.69   $269.70   $269.70  
  Purchase #5 $29.67   10.777%   $32.8675   $32.87   $32.85   $32.90  
  Purchase #6 $93.60   10.721%   $103.6349   $103.63   $103.65   $103.60  
  Purchase #7 $42.68   12.789%   $48.1383   $48.14   $48.15   $48.10  
  7 purchase totals = $817.96       $900.6422   $900.65   $900.70   $900.60