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 $5.49   4.234%   $5.7224   $5.72   $5.70   $5.70  
  Purchase #2 $207.80   9.393%   $227.3187   $227.32   $227.30   $227.30  
  Purchase #3 $80.95   4.753%   $84.7976   $84.80   $84.80   $84.80  
  Purchase #4 $197.90   10.193%   $218.0719   $218.07   $218.05   $218.10  
  Purchase #5 $93.45   5.813%   $98.8822   $98.88   $98.90   $98.90  
  Purchase #6 $171.34   5.170%   $180.1983   $180.20   $180.20   $180.20  
  Purchase #7 $162.87   7.598%   $175.2449   $175.24   $175.25   $175.20  
  7 purchase totals = $919.80       $990.2360   $990.23   $990.20   $990.20