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 $247.61   10.269%   $273.0371   $273.04   $273.05   $273.00  
  Purchase #2 $97.85   5.185%   $102.9235   $102.92   $102.90   $102.90  
  Purchase #3 $62.84   7.582%   $67.6045   $67.60   $67.60   $67.60  
  Purchase #4 $24.52   8.529%   $26.6113   $26.61   $26.60   $26.60  
  Purchase #5 $36.64   5.151%   $38.5273   $38.53   $38.55   $38.50  
  Purchase #6 $58.64   10.280%   $64.6682   $64.67   $64.65   $64.70  
  Purchase #7 $188.99   7.600%   $203.3532   $203.35   $203.35   $203.40  
  7 purchase totals = $717.09       $776.7251   $776.72   $776.70   $776.70