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 $174.60   8.739%   $189.8583   $189.86   $189.85   $189.90  
  Purchase #2 $42.80   6.566%   $45.6102   $45.61   $45.60   $45.60  
  Purchase #3 $16.54   11.234%   $18.3981   $18.40   $18.40   $18.40  
  Purchase #4 $50.99   10.144%   $56.1624   $56.16   $56.15   $56.20  
  Purchase #5 $173.45   8.605%   $188.3754   $188.38   $188.40   $188.40  
  Purchase #6 $10.93   6.203%   $11.608   $11.61   $11.60   $11.60  
  Purchase #7 $115.90   10.448%   $128.0092   $128.01   $128.00   $128.00  
  7 purchase totals = $585.21       $638.0216   $638.03   $638.00   $638.10