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 $104.96   11.554%   $117.0871   $117.09   $117.10   $117.10  
  Purchase #2 $51.55   4.622%   $53.9326   $53.93   $53.95   $53.90  
  Purchase #3 $99.56   9.856%   $109.3726   $109.37   $109.35   $109.40  
  Purchase #4 $79.41   8.229%   $85.9446   $85.94   $85.95   $85.90  
  Purchase #5 $200.61   5.361%   $211.3647   $211.36   $211.35   $211.40  
  Purchase #6 $87.88   3.792%   $91.2124   $91.21   $91.20   $91.20  
  Purchase #7 $164.44   4.115%   $171.2067   $171.21   $171.20   $171.20  
  7 purchase totals = $788.41       $840.1207   $840.11   $840.10   $840.10