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 $220.82   13.782%   $251.2534   $251.25   $251.25   $251.30  
  Purchase #2 $242.67   7.138%   $259.9918   $259.99   $260.00   $260.00  
  Purchase #3 $185.67   13.795%   $211.2832   $211.28   $211.30   $211.30  
  Purchase #4 $161.98   13.218%   $183.3905   $183.39   $183.40   $183.40  
  Purchase #5 $5.93   11.503%   $6.6121   $6.61   $6.60   $6.60  
  Purchase #6 $56.47   12.303%   $63.4175   $63.42   $63.40   $63.40  
  Purchase #7 $24.78   6.421%   $26.3711   $26.37   $26.35   $26.40  
  7 purchase totals = $898.32       $1,002.3196   $1,002.31   $1,002.30   $1,002.40