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 $235.32   13.795%   $267.7824   $267.78   $267.80   $267.80  
  Purchase #2 $249.94   3.445%   $258.5504   $258.55   $258.55   $258.60  
  Purchase #3 $198.27   11.277%   $220.6289   $220.63   $220.65   $220.60  
  Purchase #4 $85.68   11.872%   $95.8519   $95.85   $95.85   $95.90  
  Purchase #5 $11.68   13.123%   $13.2128   $13.21   $13.20   $13.20  
  Purchase #6 $191.87   9.873%   $210.8133   $210.81   $210.80   $210.80  
  Purchase #7 $182.33   4.754%   $190.998   $191.00   $191.00   $191.00  
  7 purchase totals = $1,155.09       $1,257.8377   $1,257.83   $1,257.85   $1,257.90