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 $20.36   6.441%   $21.6714   $21.67   $21.65   $21.70  
  Purchase #2 $131.52   5.191%   $138.3472   $138.35   $138.35   $138.30  
  Purchase #3 $134.67   6.278%   $143.1246   $143.12   $143.10   $143.10  
  Purchase #4 $13.42   4.494%   $14.0231   $14.02   $14.00   $14.00  
  Purchase #5 $161.81   10.203%   $178.3195   $178.32   $178.30   $178.30  
  Purchase #6 $220.73   12.702%   $248.7671   $248.77   $248.75   $248.80  
  Purchase #7 $111.30   8.585%   $120.8551   $120.86   $120.85   $120.90  
  7 purchase totals = $793.81       $865.1080   $865.11   $865.00   $865.10