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 $169.98   4.715%   $177.9946   $177.99   $178.00   $178.00  
  Purchase #2 $108.34   8.743%   $117.8122   $117.81   $117.80   $117.80  
  Purchase #3 $139.93   10.872%   $155.1432   $155.14   $155.15   $155.10  
  Purchase #4 $77.54   12.748%   $87.4248   $87.42   $87.40   $87.40  
  Purchase #5 $63.64   5.582%   $67.1924   $67.19   $67.20   $67.20  
  Purchase #6 $45.25   8.348%   $49.0275   $49.03   $49.05   $49.00  
  Purchase #7 $9.44   5.758%   $9.9836   $9.98   $10.00   $10.00  
  7 purchase totals = $614.12       $664.5783   $664.56   $664.60   $664.50