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 $128.80   3.120%   $132.8186   $132.82   $132.80   $132.80  
  Purchase #2 $28.67   5.781%   $30.3274   $30.33   $30.35   $30.30  
  Purchase #3 $110.40   9.775%   $121.1916   $121.19   $121.20   $121.20  
  Purchase #4 $35.58   13.211%   $40.2805   $40.28   $40.30   $40.30  
  Purchase #5 $31.99   12.116%   $35.8659   $35.87   $35.85   $35.90  
  Purchase #6 $50.34   6.342%   $53.5326   $53.53   $53.55   $53.50  
  Purchase #7 $218.82   10.467%   $241.7239   $241.72   $241.70   $241.70  
  7 purchase totals = $604.60       $655.7405   $655.74   $655.75   $655.70