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 $131.34   3.737%   $136.2482   $136.25   $136.25   $136.20  
  Purchase #2 $156.42   12.857%   $176.5309   $176.53   $176.55   $176.50  
  Purchase #3 $11.95   8.786%   $12.9999   $13.00   $13.00   $13.00  
  Purchase #4 $196.58   12.677%   $221.5004   $221.50   $221.50   $221.50  
  Purchase #5 $203.47   7.565%   $218.8625   $218.86   $218.85   $218.90  
  Purchase #6 $160.92   9.186%   $175.7021   $175.70   $175.70   $175.70  
  Purchase #7 $103.50   4.124%   $107.7683   $107.77   $107.75   $107.80  
  7 purchase totals = $964.18       $1,049.6123   $1,049.61   $1,049.60   $1,049.60