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 $221.83   7.778%   $239.0839   $239.08   $239.10   $239.10  
  Purchase #2 $192.72   13.333%   $218.4154   $218.42   $218.40   $218.40  
  Purchase #3 $247.54   13.113%   $279.9999   $280.00   $280.00   $280.00  
  Purchase #4 $159.41   12.104%   $178.705   $178.71   $178.70   $178.70  
  Purchase #5 $214.83   9.372%   $234.9639   $234.96   $234.95   $235.00  
  Purchase #6 $223.86   11.494%   $249.5905   $249.59   $249.60   $249.60  
  Purchase #7 $11.35   9.197%   $12.3939   $12.39   $12.40   $12.40  
  7 purchase totals = $1,271.54       $1,413.1525   $1,413.15   $1,413.15   $1,413.20