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 $240.31   7.326%   $257.9151   $257.92   $257.90   $257.90  
  Purchase #2 $40.37   8.172%   $43.669   $43.67   $43.65   $43.70  
  Purchase #3 $250.61   13.469%   $284.3647   $284.36   $284.35   $284.40  
  Purchase #4 $34.43   9.808%   $37.8069   $37.81   $37.80   $37.80  
  Purchase #5 $104.92   12.497%   $118.0319   $118.03   $118.05   $118.00  
  Purchase #6 $26.69   12.691%   $30.0772   $30.08   $30.10   $30.10  
  Purchase #7 $16.99   10.665%   $18.802   $18.80   $18.80   $18.80  
  7 purchase totals = $714.32       $790.6668   $790.67   $790.65   $790.70