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 $43.61   12.409%   $49.0216   $49.02   $49.00   $49.00  
  Purchase #2 $116.69   12.823%   $131.6532   $131.65   $131.65   $131.70  
  Purchase #3 $244.99   12.210%   $274.9033   $274.90   $274.90   $274.90  
  Purchase #4 $53.34   7.559%   $57.372   $57.37   $57.35   $57.40  
  Purchase #5 $226.90   6.753%   $242.2226   $242.22   $242.20   $242.20  
  Purchase #6 $208.81   12.317%   $234.5291   $234.53   $234.55   $234.50  
  Purchase #7 $8.42   4.521%   $8.8007   $8.80   $8.80   $8.80  
  7 purchase totals = $902.76       $998.5025   $998.49   $998.45   $998.50