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 $118.65   13.456%   $134.6155   $134.62   $134.60   $134.60  
  Purchase #2 $120.90   11.657%   $134.9933   $134.99   $135.00   $135.00  
  Purchase #3 $71.64   8.191%   $77.508   $77.51   $77.50   $77.50  
  Purchase #4 $62.82   12.415%   $70.6191   $70.62   $70.60   $70.60  
  Purchase #5 $220.83   10.375%   $243.7411   $243.74   $243.75   $243.70  
  Purchase #6 $167.99   10.499%   $185.6273   $185.63   $185.65   $185.60  
  Purchase #7 $10.42   9.287%   $11.3877   $11.39   $11.40   $11.40  
  7 purchase totals = $773.25       $858.4920   $858.50   $858.50   $858.40