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 $239.41   13.204%   $271.0217   $271.02   $271.00   $271.00  
  Purchase #2 $65.70   8.677%   $71.4008   $71.40   $71.40   $71.40  
  Purchase #3 $152.93   6.781%   $163.3002   $163.30   $163.30   $163.30  
  Purchase #4 $80.97   3.681%   $83.9505   $83.95   $83.95   $84.00  
  Purchase #5 $226.82   11.849%   $253.6959   $253.70   $253.70   $253.70  
  Purchase #6 $88.96   8.470%   $96.4949   $96.49   $96.50   $96.50  
  Purchase #7 $60.31   13.392%   $68.3867   $68.39   $68.40   $68.40  
  7 purchase totals = $915.10       $1,008.2507   $1,008.25   $1,008.25   $1,008.30