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 $231.87   10.258%   $255.6552   $255.66   $255.65   $255.70  
  Purchase #2 $165.35   12.641%   $186.2519   $186.25   $186.25   $186.30  
  Purchase #3 $111.68   6.129%   $118.5249   $118.52   $118.50   $118.50  
  Purchase #4 $235.50   12.734%   $265.4886   $265.49   $265.50   $265.50  
  Purchase #5 $51.74   12.846%   $58.3865   $58.39   $58.40   $58.40  
  Purchase #6 $200.63   13.300%   $227.3138   $227.31   $227.30   $227.30  
  Purchase #7 $142.51   9.623%   $156.2237   $156.22   $156.20   $156.20  
  7 purchase totals = $1,139.28       $1,267.8446   $1,267.84   $1,267.80   $1,267.90