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 $224.55   13.152%   $254.0828   $254.08   $254.10   $254.10  
  Purchase #2 $10.54   5.649%   $11.1354   $11.14   $11.15   $11.10  
  Purchase #3 $113.55   11.260%   $126.3357   $126.34   $126.35   $126.30  
  Purchase #4 $94.35   13.778%   $107.3495   $107.35   $107.35   $107.30  
  Purchase #5 $126.31   8.861%   $137.5023   $137.50   $137.50   $137.50  
  Purchase #6 $53.49   13.448%   $60.6833   $60.68   $60.70   $60.70  
  Purchase #7 $26.80   12.163%   $30.0597   $30.06   $30.05   $30.10  
  7 purchase totals = $649.59       $727.1487   $727.15   $727.20   $727.10