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 $55.45   6.323%   $58.9561   $58.96   $58.95   $59.00  
  Purchase #2 $90.99   13.247%   $103.0434   $103.04   $103.05   $103.00  
  Purchase #3 $49.70   10.532%   $54.9344   $54.93   $54.95   $54.90  
  Purchase #4 $33.81   10.379%   $37.3191   $37.32   $37.30   $37.30  
  Purchase #5 $169.65   13.622%   $192.7597   $192.76   $192.75   $192.80  
  Purchase #6 $187.85   13.386%   $212.9956   $213.00   $213.00   $213.00  
  Purchase #7 $221.68   10.458%   $244.8633   $244.86   $244.85   $244.90  
  7 purchase totals = $809.13       $904.8716   $904.87   $904.85   $904.90