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 $121.97   12.814%   $137.5992   $137.60   $137.60   $137.60  
  Purchase #2 $23.99   12.728%   $27.0434   $27.04   $27.05   $27.00  
  Purchase #3 $17.77   10.635%   $19.6598   $19.66   $19.65   $19.70  
  Purchase #4 $135.79   6.313%   $144.3624   $144.36   $144.35   $144.40  
  Purchase #5 $129.80   11.716%   $145.0074   $145.01   $145.00   $145.00  
  Purchase #6 $21.81   11.152%   $24.2423   $24.24   $24.25   $24.20  
  Purchase #7 $135.88   7.474%   $146.0357   $146.04   $146.05   $146.00  
  7 purchase totals = $587.01       $643.9502   $643.95   $643.95   $643.90