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 $89.40   11.292%   $99.495   $99.50   $99.50   $99.50  
  Purchase #2 $39.96   6.700%   $42.6373   $42.64   $42.65   $42.60  
  Purchase #3 $163.96   4.788%   $171.8104   $171.81   $171.80   $171.80  
  Purchase #4 $240.37   5.141%   $252.7274   $252.73   $252.75   $252.70  
  Purchase #5 $137.86   7.832%   $148.6572   $148.66   $148.65   $148.70  
  Purchase #6 $120.78   7.173%   $129.4435   $129.44   $129.45   $129.40  
  Purchase #7 $143.36   4.590%   $149.9402   $149.94   $149.95   $149.90  
  7 purchase totals = $935.69       $994.7110   $994.72   $994.75   $994.60