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 $228.96   8.753%   $249.0009   $249.00   $249.00   $249.00  
  Purchase #2 $249.97   13.321%   $283.2685   $283.27   $283.25   $283.30  
  Purchase #3 $21.57   3.687%   $22.3653   $22.37   $22.35   $22.40  
  Purchase #4 $71.80   5.811%   $75.9723   $75.97   $75.95   $76.00  
  Purchase #5 $239.93   7.116%   $257.0034   $257.00   $257.00   $257.00  
  Purchase #6 $87.89   9.654%   $96.3749   $96.37   $96.35   $96.40  
  Purchase #7 $35.43   8.328%   $38.3806   $38.38   $38.40   $38.40  
  7 purchase totals = $935.55       $1,022.3659   $1,022.36   $1,022.30   $1,022.50