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 $244.62   11.867%   $273.6491   $273.65   $273.65   $273.60  
  Purchase #2 $75.97   11.169%   $84.4551   $84.46   $84.45   $84.50  
  Purchase #3 $127.72   11.370%   $142.2418   $142.24   $142.25   $142.20  
  Purchase #4 $79.35   4.589%   $82.9914   $82.99   $83.00   $83.00  
  Purchase #5 $236.83   11.597%   $264.2952   $264.30   $264.30   $264.30  
  Purchase #6 $166.40   3.301%   $171.8929   $171.89   $171.90   $171.90  
  Purchase #7 $57.34   4.736%   $60.0556   $60.06   $60.05   $60.10  
  7 purchase totals = $988.23       $1,079.5811   $1,079.59   $1,079.60   $1,079.60