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 $249.99   10.800%   $276.9889   $276.99   $277.00   $277.00  
  Purchase #2 $37.68   11.398%   $41.9748   $41.97   $41.95   $42.00  
  Purchase #3 $58.92   10.462%   $65.0842   $65.08   $65.10   $65.10  
  Purchase #4 $94.58   7.184%   $101.3746   $101.37   $101.35   $101.40  
  Purchase #5 $131.77   10.427%   $145.5097   $145.51   $145.50   $145.50  
  Purchase #6 $167.36   12.575%   $188.4055   $188.41   $188.40   $188.40  
  Purchase #7 $168.58   3.281%   $174.1111   $174.11   $174.10   $174.10  
  7 purchase totals = $908.88       $993.4488   $993.44   $993.40   $993.50