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 $16.74   9.325%   $18.301   $18.30   $18.30   $18.30  
  Purchase #2 $184.56   10.558%   $204.0458   $204.05   $204.05   $204.00  
  Purchase #3 $109.64   10.494%   $121.1456   $121.15   $121.15   $121.10  
  Purchase #4 $124.49   5.716%   $131.6058   $131.61   $131.60   $131.60  
  Purchase #5 $76.41   12.290%   $85.8008   $85.80   $85.80   $85.80  
  Purchase #6 $21.45   8.683%   $23.3125   $23.31   $23.30   $23.30  
  Purchase #7 $55.67   4.585%   $58.2225   $58.22   $58.20   $58.20  
  7 purchase totals = $588.96       $642.4340   $642.44   $642.40   $642.30