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 $30.28   12.219%   $33.9799   $33.98   $34.00   $34.00  
  Purchase #2 $112.77   10.727%   $124.8668   $124.87   $124.85   $124.90  
  Purchase #3 $244.25   13.385%   $276.9429   $276.94   $276.95   $276.90  
  Purchase #4 $65.86   5.167%   $69.263   $69.26   $69.25   $69.30  
  Purchase #5 $228.35   8.545%   $247.8625   $247.86   $247.85   $247.90  
  Purchase #6 $86.63   10.102%   $95.3814   $95.38   $95.40   $95.40  
  Purchase #7 $17.35   3.805%   $18.0102   $18.01   $18.00   $18.00  
  7 purchase totals = $785.49       $866.3067   $866.30   $866.30   $866.40