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 $18.64   13.215%   $21.1033   $21.10   $21.10   $21.10  
  Purchase #2 $6.68   12.580%   $7.5203   $7.52   $7.50   $7.50  
  Purchase #3 $135.63   7.672%   $146.0355   $146.04   $146.05   $146.00  
  Purchase #4 $103.84   3.466%   $107.4391   $107.44   $107.45   $107.40  
  Purchase #5 $78.96   4.537%   $82.5424   $82.54   $82.55   $82.50  
  Purchase #6 $39.30   6.849%   $41.9917   $41.99   $42.00   $42.00  
  Purchase #7 $192.94   3.161%   $199.0388   $199.04   $199.05   $199.00  
  7 purchase totals = $575.99       $605.6711   $605.67   $605.70   $605.50