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 $28.99   6.372%   $30.8372   $30.84   $30.85   $30.80  
  Purchase #2 $192.77   8.614%   $209.3752   $209.38   $209.40   $209.40  
  Purchase #3 $210.86   6.278%   $224.0978   $224.10   $224.10   $224.10  
  Purchase #4 $133.43   10.772%   $147.8031   $147.80   $147.80   $147.80  
  Purchase #5 $137.55   9.677%   $150.8607   $150.86   $150.85   $150.90  
  Purchase #6 $47.39   4.580%   $49.5605   $49.56   $49.55   $49.60  
  Purchase #7 $24.39   10.844%   $27.0349   $27.03   $27.05   $27.00  
  7 purchase totals = $775.38       $839.5694   $839.57   $839.60   $839.60