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 $34.76   11.487%   $38.7529   $38.75   $38.75   $38.80  
  Purchase #2 $97.38   6.341%   $103.5549   $103.55   $103.55   $103.60  
  Purchase #3 $225.44   5.388%   $237.5867   $237.59   $237.60   $237.60  
  Purchase #4 $149.97   12.212%   $168.2843   $168.28   $168.30   $168.30  
  Purchase #5 $20.41   4.688%   $21.3668   $21.37   $21.35   $21.40  
  Purchase #6 $31.54   10.593%   $34.881   $34.88   $34.90   $34.90  
  Purchase #7 $134.48   12.312%   $151.0372   $151.04   $151.05   $151.00  
  7 purchase totals = $693.98       $755.4638   $755.46   $755.50   $755.60