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 $29.42   13.239%   $33.3149   $33.31   $33.30   $33.30  
  Purchase #2 $224.83   10.273%   $247.9268   $247.93   $247.95   $247.90  
  Purchase #3 $161.84   4.864%   $169.7119   $169.71   $169.70   $169.70  
  Purchase #4 $26.31   6.570%   $28.0386   $28.04   $28.05   $28.00  
  Purchase #5 $207.29   13.143%   $234.5341   $234.53   $234.55   $234.50  
  Purchase #6 $186.32   13.244%   $210.9962   $211.00   $211.00   $211.00  
  Purchase #7 $68.40   13.154%   $77.3973   $77.40   $77.40   $77.40  
  7 purchase totals = $904.41       $1,001.9198   $1,001.92   $1,001.95   $1,001.80