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 $91.48   9.286%   $99.9748   $99.97   $99.95   $100.00  
  Purchase #2 $17.80   7.577%   $19.1487   $19.15   $19.15   $19.10  
  Purchase #3 $236.48   5.103%   $248.5476   $248.55   $248.55   $248.50  
  Purchase #4 $28.67   8.339%   $31.0608   $31.06   $31.05   $31.10  
  Purchase #5 $141.54   4.730%   $148.2348   $148.23   $148.25   $148.20  
  Purchase #6 $14.30   7.175%   $15.326   $15.33   $15.35   $15.30  
  Purchase #7 $108.43   12.585%   $122.0759   $122.08   $122.10   $122.10  
  7 purchase totals = $638.70       $684.3686   $684.37   $684.40   $684.30