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 $238.68   12.803%   $269.2382   $269.24   $269.25   $269.20  
  Purchase #2 $29.43   9.372%   $32.1882   $32.19   $32.20   $32.20  
  Purchase #3 $151.49   7.694%   $163.1456   $163.15   $163.15   $163.10  
  Purchase #4 $76.51   11.849%   $85.5757   $85.58   $85.60   $85.60  
  Purchase #5 $101.84   10.767%   $112.8051   $112.81   $112.80   $112.80  
  Purchase #6 $56.41   8.183%   $61.026   $61.03   $61.05   $61.00  
  Purchase #7 $124.92   7.766%   $134.6213   $134.62   $134.60   $134.60  
  7 purchase totals = $779.28       $858.6001   $858.62   $858.65   $858.50