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 $136.74   12.469%   $153.7901   $153.79   $153.80   $153.80  
  Purchase #2 $217.63   3.112%   $224.4026   $224.40   $224.40   $224.40  
  Purchase #3 $227.35   12.686%   $256.1916   $256.19   $256.20   $256.20  
  Purchase #4 $84.76   12.704%   $95.5279   $95.53   $95.55   $95.50  
  Purchase #5 $100.55   13.852%   $114.4782   $114.48   $114.50   $114.50  
  Purchase #6 $82.99   5.809%   $87.8109   $87.81   $87.80   $87.80  
  Purchase #7 $126.28   6.142%   $134.0361   $134.04   $134.05   $134.00  
  7 purchase totals = $976.30       $1,066.2374   $1,066.24   $1,066.30   $1,066.20