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 $18.30   11.837%   $20.4662   $20.47   $20.45   $20.50  
  Purchase #2 $166.66   7.247%   $178.7379   $178.74   $178.75   $178.70  
  Purchase #3 $157.62   3.434%   $163.0327   $163.03   $163.05   $163.00  
  Purchase #4 $21.28   8.254%   $23.0365   $23.04   $23.05   $23.00  
  Purchase #5 $233.52   6.288%   $248.2037   $248.20   $248.20   $248.20  
  Purchase #6 $233.59   5.844%   $247.241   $247.24   $247.25   $247.20  
  Purchase #7 $192.44   13.484%   $218.3886   $218.39   $218.40   $218.40  
  7 purchase totals = $1,023.41       $1,099.1066   $1,099.11   $1,099.15   $1,099.00