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 $115.90   12.155%   $129.9876   $129.99   $130.00   $130.00  
  Purchase #2 $140.77   6.538%   $149.9735   $149.97   $149.95   $150.00  
  Purchase #3 $133.95   5.837%   $141.7687   $141.77   $141.75   $141.80  
  Purchase #4 $171.61   12.607%   $193.2449   $193.24   $193.25   $193.20  
  Purchase #5 $106.87   5.343%   $112.5801   $112.58   $112.60   $112.60  
  Purchase #6 $14.37   12.220%   $16.126   $16.13   $16.15   $16.10  
  Purchase #7 $188.63   7.550%   $202.8716   $202.87   $202.85   $202.90  
  7 purchase totals = $872.10       $946.5524   $946.55   $946.55   $946.60