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 $248.61   11.258%   $276.5985   $276.60   $276.60   $276.60  
  Purchase #2 $51.48   4.578%   $53.8368   $53.84   $53.85   $53.80  
  Purchase #3 $197.64   3.572%   $204.6997   $204.70   $204.70   $204.70  
  Purchase #4 $67.75   13.604%   $76.9667   $76.97   $76.95   $77.00  
  Purchase #5 $18.77   6.370%   $19.9656   $19.97   $19.95   $20.00  
  Purchase #6 $234.32   5.872%   $248.0793   $248.08   $248.10   $248.10  
  Purchase #7 $127.71   9.726%   $140.1311   $140.13   $140.15   $140.10  
  7 purchase totals = $946.28       $1,020.2777   $1,020.29   $1,020.30   $1,020.30