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 $137.92   8.275%   $149.3329   $149.33   $149.35   $149.30  
  Purchase #2 $67.50   12.491%   $75.9314   $75.93   $75.95   $75.90  
  Purchase #3 $133.66   12.175%   $149.9331   $149.93   $149.95   $149.90  
  Purchase #4 $34.27   12.749%   $38.6391   $38.64   $38.65   $38.60  
  Purchase #5 $121.71   10.629%   $134.6466   $134.65   $134.65   $134.60  
  Purchase #6 $228.57   8.517%   $248.0373   $248.04   $248.05   $248.00  
  Purchase #7 $108.33   6.642%   $115.5253   $115.53   $115.55   $115.50  
  7 purchase totals = $831.96       $912.0457   $912.05   $912.15   $911.80