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 $236.85   12.193%   $265.7291   $265.73   $265.75   $265.70  
  Purchase #2 $41.74   12.334%   $46.8882   $46.89   $46.90   $46.90  
  Purchase #3 $164.57   4.297%   $171.6416   $171.64   $171.65   $171.60  
  Purchase #4 $129.44   8.341%   $140.2366   $140.24   $140.25   $140.20  
  Purchase #5 $14.61   12.232%   $16.3971   $16.40   $16.40   $16.40  
  Purchase #6 $171.49   6.184%   $182.0949   $182.09   $182.10   $182.10  
  Purchase #7 $160.73   13.121%   $181.8194   $181.82   $181.80   $181.80  
  7 purchase totals = $919.43       $1,004.8069   $1,004.81   $1,004.85   $1,004.70