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 $248.90   10.745%   $275.6443   $275.64   $275.65   $275.60  
  Purchase #2 $224.66   11.112%   $249.6242   $249.62   $249.60   $249.60  
  Purchase #3 $110.27   5.581%   $116.4242   $116.42   $116.40   $116.40  
  Purchase #4 $62.89   13.417%   $71.328   $71.33   $71.35   $71.30  
  Purchase #5 $192.38   5.558%   $203.0725   $203.07   $203.05   $203.10  
  Purchase #6 $143.68   7.730%   $154.7865   $154.79   $154.80   $154.80  
  Purchase #7 $159.63   11.116%   $177.3745   $177.37   $177.35   $177.40  
  7 purchase totals = $1,142.41       $1,248.2542   $1,248.24   $1,248.20   $1,248.20