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 $217.79   13.273%   $246.6973   $246.70   $246.70   $246.70  
  Purchase #2 $137.53   11.315%   $153.0915   $153.09   $153.10   $153.10  
  Purchase #3 $87.43   10.135%   $96.291   $96.29   $96.30   $96.30  
  Purchase #4 $201.97   3.302%   $208.639   $208.64   $208.65   $208.60  
  Purchase #5 $203.87   12.176%   $228.6932   $228.69   $228.70   $228.70  
  Purchase #6 $114.68   7.265%   $123.0115   $123.01   $123.00   $123.00  
  Purchase #7 $215.86   3.870%   $224.2138   $224.21   $224.20   $224.20  
  7 purchase totals = $1,179.13       $1,280.6373   $1,280.63   $1,280.65   $1,280.60