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 $80.32   7.542%   $86.3777   $86.38   $86.40   $86.40  
  Purchase #2 $5.63   3.830%   $5.8456   $5.85   $5.85   $5.80  
  Purchase #3 $18.83   6.404%   $20.0359   $20.04   $20.05   $20.00  
  Purchase #4 $239.42   12.757%   $269.9628   $269.96   $269.95   $270.00  
  Purchase #5 $25.43   13.821%   $28.9447   $28.94   $28.95   $28.90  
  Purchase #6 $196.60   9.608%   $215.4893   $215.49   $215.50   $215.50  
  Purchase #7 $142.45   12.390%   $160.0996   $160.10   $160.10   $160.10  
  7 purchase totals = $708.68       $786.7556   $786.76   $786.80   $786.70