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 $6.62   13.344%   $7.5034   $7.50   $7.50   $7.50  
  Purchase #2 $164.37   3.626%   $170.3301   $170.33   $170.35   $170.30  
  Purchase #3 $160.69   3.653%   $166.56   $166.56   $166.55   $166.60  
  Purchase #4 $74.94   4.323%   $78.1797   $78.18   $78.20   $78.20  
  Purchase #5 $107.61   13.468%   $122.1029   $122.10   $122.10   $122.10  
  Purchase #6 $239.50   11.726%   $267.5838   $267.58   $267.60   $267.60  
  Purchase #7 $187.99   7.618%   $202.3111   $202.31   $202.30   $202.30  
  7 purchase totals = $941.72       $1,014.5710   $1,014.56   $1,014.60   $1,014.60