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 $228.46   13.646%   $259.6357   $259.64   $259.65   $259.60  
  Purchase #2 $55.43   4.258%   $57.7902   $57.79   $57.80   $57.80  
  Purchase #3 $226.57   11.419%   $252.442   $252.44   $252.45   $252.40  
  Purchase #4 $241.25   9.205%   $263.4571   $263.46   $263.45   $263.50  
  Purchase #5 $147.25   8.499%   $159.7648   $159.76   $159.75   $159.80  
  Purchase #6 $27.80   7.704%   $29.9417   $29.94   $29.95   $29.90  
  Purchase #7 $241.41   3.128%   $248.9613   $248.96   $248.95   $249.00  
  7 purchase totals = $1,168.17       $1,271.9928   $1,271.99   $1,272.00   $1,272.00