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 $239.33   12.787%   $269.9331   $269.93   $269.95   $269.90  
  Purchase #2 $5.60   9.826%   $6.1503   $6.15   $6.15   $6.20  
  Purchase #3 $66.84   12.788%   $75.3875   $75.39   $75.40   $75.40  
  Purchase #4 $143.41   9.493%   $157.0239   $157.02   $157.00   $157.00  
  Purchase #5 $102.41   5.135%   $107.6688   $107.67   $107.65   $107.70  
  Purchase #6 $124.69   6.525%   $132.826   $132.83   $132.85   $132.80  
  Purchase #7 $226.67   11.519%   $252.7801   $252.78   $252.80   $252.80  
  7 purchase totals = $908.95       $1,001.7697   $1,001.77   $1,001.80   $1,001.80