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 $74.99   12.597%   $84.4365   $84.44   $84.45   $84.40  
  Purchase #2 $39.82   3.153%   $41.0755   $41.08   $41.10   $41.10  
  Purchase #3 $180.25   10.164%   $198.5706   $198.57   $198.55   $198.60  
  Purchase #4 $51.69   10.824%   $57.2849   $57.28   $57.30   $57.30  
  Purchase #5 $65.65   5.702%   $69.3934   $69.39   $69.40   $69.40  
  Purchase #6 $41.75   7.444%   $44.8579   $44.86   $44.85   $44.90  
  Purchase #7 $249.39   9.496%   $273.0721   $273.07   $273.05   $273.10  
  7 purchase totals = $703.54       $768.6909   $768.69   $768.70   $768.80