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 $56.39   12.207%   $63.2735   $63.27   $63.25   $63.30  
  Purchase #2 $166.89   7.580%   $179.5403   $179.54   $179.55   $179.50  
  Purchase #3 $74.43   11.705%   $83.142   $83.14   $83.15   $83.10  
  Purchase #4 $162.45   13.702%   $184.7089   $184.71   $184.70   $184.70  
  Purchase #5 $212.42   8.842%   $231.2022   $231.20   $231.20   $231.20  
  Purchase #6 $22.65   6.194%   $24.0529   $24.05   $24.05   $24.10  
  Purchase #7 $62.82   6.452%   $66.8731   $66.87   $66.85   $66.90  
  7 purchase totals = $758.05       $832.7929   $832.78   $832.75   $832.80