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 $18.82   9.640%   $20.6342   $20.63   $20.65   $20.60  
  Purchase #2 $57.90   12.505%   $65.1404   $65.14   $65.15   $65.10  
  Purchase #3 $14.79   5.610%   $15.6197   $15.62   $15.60   $15.60  
  Purchase #4 $231.75   13.624%   $263.3236   $263.32   $263.30   $263.30  
  Purchase #5 $122.88   9.682%   $134.7772   $134.78   $134.80   $134.80  
  Purchase #6 $138.61   6.679%   $147.8678   $147.87   $147.85   $147.90  
  Purchase #7 $107.69   12.407%   $121.0511   $121.05   $121.05   $121.10  
  7 purchase totals = $692.44       $768.4140   $768.41   $768.40   $768.40