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 $56.45   6.846%   $60.3146   $60.31   $60.30   $60.30  
  Purchase #2 $244.92   5.796%   $259.1156   $259.12   $259.10   $259.10  
  Purchase #3 $134.99   13.789%   $153.6038   $153.60   $153.60   $153.60  
  Purchase #4 $60.78   10.325%   $67.0555   $67.06   $67.05   $67.10  
  Purchase #5 $11.97   13.107%   $13.5389   $13.54   $13.55   $13.50  
  Purchase #6 $193.34   12.714%   $217.9212   $217.92   $217.90   $217.90  
  Purchase #7 $156.70   13.406%   $177.7072   $177.71   $177.70   $177.70  
  7 purchase totals = $859.15       $949.2568   $949.26   $949.20   $949.20