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 $13.39   3.406%   $13.8461   $13.85   $13.85   $13.80  
  Purchase #2 $123.40   7.847%   $133.0832   $133.08   $133.10   $133.10  
  Purchase #3 $115.95   12.503%   $130.4472   $130.45   $130.45   $130.40  
  Purchase #4 $114.53   10.813%   $126.9141   $126.91   $126.90   $126.90  
  Purchase #5 $112.42   6.853%   $120.1241   $120.12   $120.10   $120.10  
  Purchase #6 $225.52   10.327%   $248.8095   $248.81   $248.80   $248.80  
  Purchase #7 $196.70   11.871%   $220.0503   $220.05   $220.05   $220.10  
  7 purchase totals = $901.91       $993.2745   $993.27   $993.25   $993.20