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 $30.42   13.217%   $34.4406   $34.44   $34.45   $34.40  
  Purchase #2 $122.99   6.108%   $130.5022   $130.50   $130.50   $130.50  
  Purchase #3 $100.35   12.214%   $112.6067   $112.61   $112.60   $112.60  
  Purchase #4 $144.76   11.231%   $161.018   $161.02   $161.00   $161.00  
  Purchase #5 $140.33   5.427%   $147.9457   $147.95   $147.95   $147.90  
  Purchase #6 $116.98   12.422%   $131.5113   $131.51   $131.50   $131.50  
  Purchase #7 $193.33   6.687%   $206.258   $206.26   $206.25   $206.30  
  7 purchase totals = $849.16       $924.2825   $924.29   $924.25   $924.20