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 $248.36   11.446%   $276.7873   $276.79   $276.80   $276.80  
  Purchase #2 $105.41   7.397%   $113.2072   $113.21   $113.20   $113.20  
  Purchase #3 $14.62   11.804%   $16.3457   $16.35   $16.35   $16.30  
  Purchase #4 $10.86   13.579%   $12.3347   $12.33   $12.35   $12.30  
  Purchase #5 $107.71   10.220%   $118.718   $118.72   $118.70   $118.70  
  Purchase #6 $87.56   11.241%   $97.4026   $97.40   $97.40   $97.40  
  Purchase #7 $43.88   12.348%   $49.2983   $49.30   $49.30   $49.30  
  7 purchase totals = $618.40       $684.0938   $684.10   $684.10   $684.00