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 $120.55   12.835%   $136.0226   $136.02   $136.00   $136.00  
  Purchase #2 $191.35   8.802%   $208.1926   $208.19   $208.20   $208.20  
  Purchase #3 $23.37   12.456%   $26.281   $26.28   $26.30   $26.30  
  Purchase #4 $172.41   10.236%   $190.0579   $190.06   $190.05   $190.10  
  Purchase #5 $221.50   10.798%   $245.4176   $245.42   $245.40   $245.40  
  Purchase #6 $65.84   9.105%   $71.8347   $71.83   $71.85   $71.80  
  Purchase #7 $124.40   8.480%   $134.9491   $134.95   $134.95   $134.90  
  7 purchase totals = $919.42       $1,012.7555   $1,012.75   $1,012.75   $1,012.70