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 $183.70   12.142%   $206.0049   $206.00   $206.00   $206.00  
  Purchase #2 $236.79   7.633%   $254.8642   $254.86   $254.85   $254.90  
  Purchase #3 $122.93   11.269%   $136.783   $136.78   $136.80   $136.80  
  Purchase #4 $246.95   11.132%   $274.4405   $274.44   $274.45   $274.40  
  Purchase #5 $103.95   12.314%   $116.7504   $116.75   $116.75   $116.80  
  Purchase #6 $128.70   13.191%   $145.6768   $145.68   $145.70   $145.70  
  Purchase #7 $126.32   12.416%   $142.0039   $142.00   $142.00   $142.00  
  7 purchase totals = $1,149.34       $1,276.5237   $1,276.51   $1,276.55   $1,276.60