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 $28.42   13.215%   $32.1757   $32.18   $32.20   $32.20  
  Purchase #2 $21.85   9.534%   $23.9332   $23.93   $23.95   $23.90  
  Purchase #3 $103.89   5.259%   $109.3536   $109.35   $109.35   $109.40  
  Purchase #4 $161.59   11.197%   $179.6832   $179.68   $179.70   $179.70  
  Purchase #5 $34.38   10.316%   $37.9266   $37.93   $37.95   $37.90  
  Purchase #6 $109.94   8.165%   $118.9166   $118.92   $118.90   $118.90  
  Purchase #7 $126.27   9.333%   $138.0548   $138.05   $138.05   $138.10  
  7 purchase totals = $586.34       $640.0437   $640.04   $640.10   $640.10