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 $202.87   12.485%   $228.1983   $228.20   $228.20   $228.20  
  Purchase #2 $14.80   10.450%   $16.3466   $16.35   $16.35   $16.30  
  Purchase #3 $221.83   6.724%   $236.7458   $236.75   $236.75   $236.70  
  Purchase #4 $87.46   13.118%   $98.933   $98.93   $98.95   $98.90  
  Purchase #5 $7.58   3.584%   $7.8517   $7.85   $7.85   $7.90  
  Purchase #6 $169.95   8.411%   $184.2445   $184.24   $184.25   $184.20  
  Purchase #7 $52.25   5.169%   $54.9508   $54.95   $54.95   $55.00  
  7 purchase totals = $756.74       $827.2707   $827.27   $827.30   $827.20