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 $180.38   4.439%   $188.3871   $188.39   $188.40   $188.40  
  Purchase #2 $133.67   11.874%   $149.542   $149.54   $149.55   $149.50  
  Purchase #3 $154.66   10.172%   $170.392   $170.39   $170.40   $170.40  
  Purchase #4 $198.84   8.554%   $215.8488   $215.85   $215.85   $215.80  
  Purchase #5 $52.41   13.533%   $59.5026   $59.50   $59.50   $59.50  
  Purchase #6 $184.58   10.839%   $204.5866   $204.59   $204.60   $204.60  
  Purchase #7 $9.66   5.787%   $10.219   $10.22   $10.20   $10.20  
  7 purchase totals = $914.20       $998.4781   $998.48   $998.50   $998.40