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 $28.75   4.539%   $30.055   $30.06   $30.05   $30.10  
  Purchase #2 $109.88   9.319%   $120.1197   $120.12   $120.10   $120.10  
  Purchase #3 $7.53   13.162%   $8.5211   $8.52   $8.50   $8.50  
  Purchase #4 $115.50   7.759%   $124.4616   $124.46   $124.45   $124.50  
  Purchase #5 $187.97   10.609%   $207.9117   $207.91   $207.90   $207.90  
  Purchase #6 $194.50   8.113%   $210.2798   $210.28   $210.30   $210.30  
  Purchase #7 $183.34   13.126%   $207.4052   $207.41   $207.40   $207.40  
  7 purchase totals = $827.47       $908.7541   $908.76   $908.70   $908.80