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 $34.51   10.651%   $38.1857   $38.19   $38.20   $38.20  
  Purchase #2 $217.38   9.345%   $237.6942   $237.69   $237.70   $237.70  
  Purchase #3 $97.93   10.590%   $108.3008   $108.30   $108.30   $108.30  
  Purchase #4 $60.47   10.411%   $66.7655   $66.77   $66.75   $66.80  
  Purchase #5 $248.94   12.666%   $280.4707   $280.47   $280.45   $280.50  
  Purchase #6 $241.75   8.801%   $263.0264   $263.03   $263.05   $263.00  
  Purchase #7 $220.73   12.287%   $247.8511   $247.85   $247.85   $247.90  
  7 purchase totals = $1,121.71       $1,242.2944   $1,242.30   $1,242.30   $1,242.40