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 $231.35   11.493%   $257.9391   $257.94   $257.95   $257.90  
  Purchase #2 $191.76   5.117%   $201.5724   $201.57   $201.55   $201.60  
  Purchase #3 $169.88   5.112%   $178.5643   $178.56   $178.55   $178.60  
  Purchase #4 $31.57   7.591%   $33.9665   $33.97   $33.95   $34.00  
  Purchase #5 $121.48   11.574%   $135.5401   $135.54   $135.55   $135.50  
  Purchase #6 $38.81   9.277%   $42.4104   $42.41   $42.40   $42.40  
  Purchase #7 $6.27   3.800%   $6.5083   $6.51   $6.50   $6.50  
  7 purchase totals = $791.12       $856.5011   $856.50   $856.45   $856.50