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 $109.87   10.595%   $121.5107   $121.51   $121.50   $121.50  
  Purchase #2 $186.67   11.240%   $207.6517   $207.65   $207.65   $207.70  
  Purchase #3 $167.65   12.284%   $188.2441   $188.24   $188.25   $188.20  
  Purchase #4 $175.79   4.594%   $183.8658   $183.87   $183.85   $183.90  
  Purchase #5 $133.61   13.302%   $151.3828   $151.38   $151.40   $151.40  
  Purchase #6 $165.28   13.769%   $188.0374   $188.04   $188.05   $188.00  
  Purchase #7 $28.79   13.863%   $32.7812   $32.78   $32.80   $32.80  
  7 purchase totals = $967.66       $1,073.4737   $1,073.47   $1,073.50   $1,073.50