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 $28.57   5.321%   $30.0902   $30.09   $30.10   $30.10  
  Purchase #2 $12.54   3.719%   $13.0064   $13.01   $13.00   $13.00  
  Purchase #3 $159.53   8.743%   $173.4777   $173.48   $173.50   $173.50  
  Purchase #4 $202.85   7.126%   $217.3051   $217.31   $217.30   $217.30  
  Purchase #5 $13.79   9.216%   $15.0609   $15.06   $15.05   $15.10  
  Purchase #6 $79.45   13.590%   $90.2473   $90.25   $90.25   $90.20  
  Purchase #7 $42.38   12.857%   $47.8288   $47.83   $47.85   $47.80  
  7 purchase totals = $539.11       $587.0164   $587.03   $587.05   $587.00