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 $222.90   11.704%   $248.9882   $248.99   $249.00   $249.00  
  Purchase #2 $11.45   5.751%   $12.1085   $12.11   $12.10   $12.10  
  Purchase #3 $157.52   6.634%   $167.9699   $167.97   $167.95   $168.00  
  Purchase #4 $212.75   3.789%   $220.8111   $220.81   $220.80   $220.80  
  Purchase #5 $107.75   9.759%   $118.2653   $118.27   $118.25   $118.30  
  Purchase #6 $124.47   9.416%   $136.1901   $136.19   $136.20   $136.20  
  Purchase #7 $105.43   8.769%   $114.6752   $114.68   $114.70   $114.70  
  7 purchase totals = $942.27       $1,019.0083   $1,019.02   $1,019.00   $1,019.10