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 $50.72   12.189%   $56.9023   $56.90   $56.90   $56.90  
  Purchase #2 $173.45   10.538%   $191.7282   $191.73   $191.75   $191.70  
  Purchase #3 $232.59   3.244%   $240.1352   $240.14   $240.15   $240.10  
  Purchase #4 $110.98   4.463%   $115.933   $115.93   $115.95   $115.90  
  Purchase #5 $17.69   8.765%   $19.2405   $19.24   $19.25   $19.20  
  Purchase #6 $160.26   7.388%   $172.1   $172.10   $172.10   $172.10  
  Purchase #7 $145.99   7.732%   $157.2779   $157.28   $157.30   $157.30  
  7 purchase totals = $891.68       $953.3171   $953.32   $953.40   $953.20