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 $79.99   13.639%   $90.8998   $90.90   $90.90   $90.90  
  Purchase #2 $197.46   8.326%   $213.9005   $213.90   $213.90   $213.90  
  Purchase #3 $51.46   6.764%   $54.9408   $54.94   $54.95   $54.90  
  Purchase #4 $201.76   3.781%   $209.3885   $209.39   $209.40   $209.40  
  Purchase #5 $77.75   4.682%   $81.3903   $81.39   $81.40   $81.40  
  Purchase #6 $177.61   9.474%   $194.4368   $194.44   $194.45   $194.40  
  Purchase #7 $214.76   10.231%   $236.7321   $236.73   $236.75   $236.70  
  7 purchase totals = $1,000.79       $1,081.6888   $1,081.69   $1,081.75   $1,081.60