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 $62.48   12.415%   $70.2369   $70.24   $70.25   $70.20  
  Purchase #2 $28.75   3.164%   $29.6597   $29.66   $29.65   $29.70  
  Purchase #3 $62.87   5.210%   $66.1455   $66.15   $66.15   $66.10  
  Purchase #4 $199.93   12.546%   $225.0132   $225.01   $225.00   $225.00  
  Purchase #5 $11.68   4.418%   $12.196   $12.20   $12.20   $12.20  
  Purchase #6 $56.66   7.102%   $60.684   $60.68   $60.70   $60.70  
  Purchase #7 $97.79   7.548%   $105.1712   $105.17   $105.15   $105.20  
  7 purchase totals = $520.16       $569.1065   $569.11   $569.10   $569.10