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 $249.94   8.632%   $271.5148   $271.51   $271.50   $271.50  
  Purchase #2 $45.38   11.620%   $50.6532   $50.65   $50.65   $50.70  
  Purchase #3 $239.42   6.550%   $255.102   $255.10   $255.10   $255.10  
  Purchase #4 $212.67   12.232%   $238.6838   $238.68   $238.70   $238.70  
  Purchase #5 $97.73   8.441%   $105.9794   $105.98   $106.00   $106.00  
  Purchase #6 $10.89   12.398%   $12.2401   $12.24   $12.25   $12.20  
  Purchase #7 $77.88   13.832%   $88.6524   $88.65   $88.65   $88.70  
  7 purchase totals = $933.91       $1,022.8257   $1,022.81   $1,022.85   $1,022.90