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 $247.78   12.214%   $278.0438   $278.04   $278.05   $278.00  
  Purchase #2 $174.38   8.408%   $189.0419   $189.04   $189.05   $189.00  
  Purchase #3 $114.32   4.418%   $119.3707   $119.37   $119.35   $119.40  
  Purchase #4 $24.50   9.140%   $26.7393   $26.74   $26.75   $26.70  
  Purchase #5 $72.88   13.117%   $82.4397   $82.44   $82.45   $82.40  
  Purchase #6 $248.35   9.683%   $272.3977   $272.40   $272.40   $272.40  
  Purchase #7 $239.41   11.602%   $267.1863   $267.19   $267.20   $267.20  
  7 purchase totals = $1,121.62       $1,235.2194   $1,235.22   $1,235.25   $1,235.10