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 $217.97   13.760%   $247.9627   $247.96   $247.95   $248.00  
  Purchase #2 $122.38   12.738%   $137.9688   $137.97   $137.95   $138.00  
  Purchase #3 $70.25   6.619%   $74.8998   $74.90   $74.90   $74.90  
  Purchase #4 $217.79   6.469%   $231.8788   $231.88   $231.90   $231.90  
  Purchase #5 $211.81   11.599%   $236.3778   $236.38   $236.40   $236.40  
  Purchase #6 $248.98   10.334%   $274.7096   $274.71   $274.70   $274.70  
  Purchase #7 $86.63   3.668%   $89.8076   $89.81   $89.80   $89.80  
  7 purchase totals = $1,175.81       $1,293.6051   $1,293.61   $1,293.60   $1,293.70