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 $135.36   3.360%   $139.9081   $139.91   $139.90   $139.90  
  Purchase #2 $147.43   6.555%   $157.094   $157.09   $157.10   $157.10  
  Purchase #3 $131.78   6.302%   $140.0848   $140.08   $140.10   $140.10  
  Purchase #4 $100.69   5.395%   $106.1222   $106.12   $106.10   $106.10  
  Purchase #5 $115.32   9.700%   $126.506   $126.51   $126.50   $126.50  
  Purchase #6 $247.33   13.298%   $280.2199   $280.22   $280.20   $280.20  
  Purchase #7 $221.42   13.359%   $250.9995   $251.00   $251.00   $251.00  
  7 purchase totals = $1,099.33       $1,200.9345   $1,200.93   $1,200.90   $1,200.90