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 $231.34   11.871%   $258.8024   $258.80   $258.80   $258.80  
  Purchase #2 $179.62   6.365%   $191.0528   $191.05   $191.05   $191.10  
  Purchase #3 $178.77   3.812%   $185.5847   $185.58   $185.60   $185.60  
  Purchase #4 $237.53   3.687%   $246.2877   $246.29   $246.30   $246.30  
  Purchase #5 $22.41   8.199%   $24.2474   $24.25   $24.25   $24.20  
  Purchase #6 $165.55   5.225%   $174.2   $174.20   $174.20   $174.20  
  Purchase #7 $193.93   6.609%   $206.7468   $206.75   $206.75   $206.70  
  7 purchase totals = $1,209.15       $1,286.9218   $1,286.92   $1,286.95   $1,286.90