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 $246.43   13.482%   $279.6537   $279.65   $279.65   $279.70  
  Purchase #2 $139.89   10.526%   $154.6148   $154.61   $154.60   $154.60  
  Purchase #3 $66.28   12.635%   $74.6545   $74.65   $74.65   $74.70  
  Purchase #4 $52.36   12.489%   $58.8992   $58.90   $58.90   $58.90  
  Purchase #5 $193.53   12.319%   $217.371   $217.37   $217.35   $217.40  
  Purchase #6 $162.87   7.327%   $174.8035   $174.80   $174.80   $174.80  
  Purchase #7 $222.67   8.323%   $241.2028   $241.20   $241.20   $241.20  
  7 purchase totals = $1,084.03       $1,201.1995   $1,201.18   $1,201.15   $1,201.30