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 $21.57   4.282%   $22.4936   $22.49   $22.50   $22.50  
  Purchase #2 $17.81   10.805%   $19.7344   $19.73   $19.75   $19.70  
  Purchase #3 $142.32   12.841%   $160.5953   $160.60   $160.60   $160.60  
  Purchase #4 $10.27   8.203%   $11.1124   $11.11   $11.10   $11.10  
  Purchase #5 $32.71   9.295%   $35.7504   $35.75   $35.75   $35.80  
  Purchase #6 $185.51   10.380%   $204.7659   $204.77   $204.75   $204.80  
  Purchase #7 $61.51   11.209%   $68.4047   $68.40   $68.40   $68.40  
  7 purchase totals = $471.70       $522.8567   $522.85   $522.85   $522.90