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 $23.95   9.826%   $26.3033   $26.30   $26.30   $26.30  
  Purchase #2 $126.41   9.119%   $137.9373   $137.94   $137.95   $137.90  
  Purchase #3 $133.53   8.852%   $145.3501   $145.35   $145.35   $145.40  
  Purchase #4 $151.32   7.623%   $162.8551   $162.86   $162.85   $162.90  
  Purchase #5 $89.39   10.745%   $98.995   $99.00   $99.00   $99.00  
  Purchase #6 $120.95   11.579%   $134.9548   $134.95   $134.95   $135.00  
  Purchase #7 $133.78   12.227%   $150.1373   $150.14   $150.15   $150.10  
  7 purchase totals = $779.33       $856.5329   $856.54   $856.55   $856.60