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 $249.64   13.364%   $283.0019   $283.00   $283.00   $283.00  
  Purchase #2 $154.96   5.185%   $162.9947   $162.99   $163.00   $163.00  
  Purchase #3 $48.43   10.209%   $53.3742   $53.37   $53.35   $53.40  
  Purchase #4 $142.74   10.787%   $158.1374   $158.14   $158.15   $158.10  
  Purchase #5 $109.83   12.559%   $123.6235   $123.62   $123.60   $123.60  
  Purchase #6 $9.79   4.281%   $10.2091   $10.21   $10.20   $10.20  
  Purchase #7 $11.65   8.295%   $12.6164   $12.62   $12.60   $12.60  
  7 purchase totals = $727.04       $803.9572   $803.95   $803.90   $803.90