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 $19.30   12.830%   $21.7762   $21.78   $21.80   $21.80  
  Purchase #2 $6.54   13.685%   $7.435   $7.44   $7.45   $7.40  
  Purchase #3 $147.74   13.820%   $168.1577   $168.16   $168.15   $168.20  
  Purchase #4 $185.97   10.341%   $205.2012   $205.20   $205.20   $205.20  
  Purchase #5 $126.58   13.617%   $143.8164   $143.82   $143.80   $143.80  
  Purchase #6 $161.91   10.163%   $178.3649   $178.36   $178.35   $178.40  
  Purchase #7 $91.43   6.562%   $97.4296   $97.43   $97.45   $97.40  
  7 purchase totals = $739.47       $822.1810   $822.19   $822.20   $822.20