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 $248.46   11.501%   $277.0354   $277.04   $277.05   $277.00  
  Purchase #2 $218.60   7.399%   $234.7742   $234.77   $234.75   $234.80  
  Purchase #3 $40.44   13.436%   $45.8735   $45.87   $45.85   $45.90  
  Purchase #4 $25.41   11.238%   $28.2656   $28.27   $28.25   $28.30  
  Purchase #5 $70.70   7.448%   $75.9657   $75.97   $75.95   $76.00  
  Purchase #6 $53.48   9.492%   $58.5563   $58.56   $58.55   $58.60  
  Purchase #7 $226.34   4.170%   $235.7784   $235.78   $235.80   $235.80  
  7 purchase totals = $883.43       $956.2491   $956.26   $956.20   $956.40