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 $241.65   6.459%   $257.2582   $257.26   $257.25   $257.30  
  Purchase #2 $67.93   5.532%   $71.6879   $71.69   $71.70   $71.70  
  Purchase #3 $27.55   12.247%   $30.924   $30.92   $30.90   $30.90  
  Purchase #4 $172.75   6.706%   $184.3346   $184.33   $184.35   $184.30  
  Purchase #5 $88.87   10.824%   $98.4893   $98.49   $98.50   $98.50  
  Purchase #6 $112.86   13.397%   $127.9799   $127.98   $128.00   $128.00  
  Purchase #7 $205.28   10.290%   $226.4033   $226.40   $226.40   $226.40  
  7 purchase totals = $916.89       $997.0772   $997.07   $997.10   $997.10