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 $19.51   3.870%   $20.265   $20.27   $20.25   $20.30  
  Purchase #2 $80.72   12.776%   $91.0328   $91.03   $91.05   $91.00  
  Purchase #3 $44.70   11.491%   $49.8365   $49.84   $49.85   $49.80  
  Purchase #4 $145.68   3.598%   $150.9216   $150.92   $150.90   $150.90  
  Purchase #5 $28.44   12.410%   $31.9694   $31.97   $31.95   $32.00  
  Purchase #6 $110.97   9.722%   $121.7585   $121.76   $121.75   $121.80  
  Purchase #7 $73.61   12.409%   $82.7443   $82.74   $82.75   $82.70  
  7 purchase totals = $503.63       $548.5281   $548.53   $548.50   $548.50