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 $18.28   9.425%   $20.0029   $20.00   $20.00   $20.00  
  Purchase #2 $8.76   8.127%   $9.4719   $9.47   $9.45   $9.50  
  Purchase #3 $15.41   9.727%   $16.9089   $16.91   $16.90   $16.90  
  Purchase #4 $191.33   9.634%   $209.7627   $209.76   $209.75   $209.80  
  Purchase #5 $26.67   8.475%   $28.9303   $28.93   $28.95   $28.90  
  Purchase #6 $26.80   8.846%   $29.1707   $29.17   $29.15   $29.20  
  Purchase #7 $192.76   13.312%   $218.4202   $218.42   $218.40   $218.40  
  7 purchase totals = $480.01       $532.6676   $532.66   $532.60   $532.70