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 $248.97   13.848%   $283.4474   $283.45   $283.45   $283.40  
  Purchase #2 $221.41   12.797%   $249.7438   $249.74   $249.75   $249.70  
  Purchase #3 $185.32   10.203%   $204.2282   $204.23   $204.25   $204.20  
  Purchase #4 $217.44   11.562%   $242.5804   $242.58   $242.60   $242.60  
  Purchase #5 $146.64   13.407%   $166.3   $166.30   $166.30   $166.30  
  Purchase #6 $248.74   12.836%   $280.6683   $280.67   $280.65   $280.70  
  Purchase #7 $126.28   10.219%   $139.1846   $139.18   $139.20   $139.20  
  7 purchase totals = $1,394.80       $1,566.1527   $1,566.15   $1,566.20   $1,566.10