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 $245.79   12.543%   $276.6194   $276.62   $276.60   $276.60  
  Purchase #2 $75.98   8.397%   $82.36   $82.36   $82.35   $82.40  
  Purchase #3 $86.58   8.283%   $93.7514   $93.75   $93.75   $93.80  
  Purchase #4 $132.97   13.566%   $151.0087   $151.01   $151.00   $151.00  
  Purchase #5 $156.28   9.543%   $171.1938   $171.19   $171.20   $171.20  
  Purchase #6 $33.57   8.841%   $36.5379   $36.54   $36.55   $36.50  
  Purchase #7 $144.99   11.829%   $162.1409   $162.14   $162.15   $162.10  
  7 purchase totals = $876.16       $973.6121   $973.61   $973.60   $973.60