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 $18.96   7.569%   $20.3951   $20.40   $20.40   $20.40  
  Purchase #2 $19.30   6.521%   $20.5586   $20.56   $20.55   $20.60  
  Purchase #3 $92.65   10.671%   $102.5367   $102.54   $102.55   $102.50  
  Purchase #4 $115.81   9.155%   $126.4124   $126.41   $126.40   $126.40  
  Purchase #5 $128.35   11.832%   $143.5364   $143.54   $143.55   $143.50  
  Purchase #6 $224.83   12.179%   $252.212   $252.21   $252.20   $252.20  
  Purchase #7 $124.98   12.381%   $140.4538   $140.45   $140.45   $140.50  
  7 purchase totals = $724.88       $806.1050   $806.11   $806.10   $806.10