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 $123.27   4.411%   $128.7074   $128.71   $128.70   $128.70  
  Purchase #2 $13.30   8.345%   $14.4099   $14.41   $14.40   $14.40  
  Purchase #3 $171.47   9.212%   $187.2658   $187.27   $187.25   $187.30  
  Purchase #4 $6.77   8.202%   $7.3253   $7.33   $7.35   $7.30  
  Purchase #5 $80.34   6.149%   $85.2801   $85.28   $85.30   $85.30  
  Purchase #6 $77.40   12.305%   $86.9241   $86.92   $86.90   $86.90  
  Purchase #7 $118.70   9.290%   $129.7272   $129.73   $129.75   $129.70  
  7 purchase totals = $591.25       $639.6398   $639.65   $639.65   $639.60