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 $243.26   13.466%   $276.0174   $276.02   $276.00   $276.00  
  Purchase #2 $101.87   3.450%   $105.3845   $105.38   $105.40   $105.40  
  Purchase #3 $154.34   3.535%   $159.7959   $159.80   $159.80   $159.80  
  Purchase #4 $238.76   5.677%   $252.3144   $252.31   $252.30   $252.30  
  Purchase #5 $185.37   9.557%   $203.0858   $203.09   $203.10   $203.10  
  Purchase #6 $232.69   9.356%   $254.4605   $254.46   $254.45   $254.50  
  Purchase #7 $94.84   5.236%   $99.8058   $99.81   $99.80   $99.80  
  7 purchase totals = $1,251.13       $1,350.8643   $1,350.87   $1,350.85   $1,350.90