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 $232.74   8.589%   $252.73   $252.73   $252.75   $252.70  
  Purchase #2 $45.77   8.189%   $49.5181   $49.52   $49.50   $49.50  
  Purchase #3 $57.39   13.206%   $64.9689   $64.97   $64.95   $65.00  
  Purchase #4 $16.68   11.467%   $18.5927   $18.59   $18.60   $18.60  
  Purchase #5 $181.81   3.631%   $188.4115   $188.41   $188.40   $188.40  
  Purchase #6 $71.25   7.565%   $76.6401   $76.64   $76.65   $76.60  
  Purchase #7 $183.25   3.369%   $189.4237   $189.42   $189.40   $189.40  
  7 purchase totals = $788.89       $840.2850   $840.28   $840.25   $840.20