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 $163.90   12.162%   $183.8335   $183.83   $183.85   $183.80  
  Purchase #2 $28.79   13.605%   $32.7069   $32.71   $32.70   $32.70  
  Purchase #3 $201.42   3.429%   $208.3267   $208.33   $208.35   $208.30  
  Purchase #4 $107.95   9.492%   $118.1966   $118.20   $118.20   $118.20  
  Purchase #5 $178.99   3.377%   $185.0345   $185.03   $185.05   $185.00  
  Purchase #6 $169.46   3.225%   $174.9251   $174.93   $174.95   $174.90  
  Purchase #7 $130.44   4.587%   $136.4233   $136.42   $136.40   $136.40  
  7 purchase totals = $980.95       $1,039.4466   $1,039.45   $1,039.50   $1,039.30