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 $26.37   13.625%   $29.9629   $29.96   $29.95   $30.00  
  Purchase #2 $12.85   6.304%   $13.6601   $13.66   $13.65   $13.70  
  Purchase #3 $161.95   9.741%   $177.7255   $177.73   $177.75   $177.70  
  Purchase #4 $33.56   9.189%   $36.6438   $36.64   $36.65   $36.60  
  Purchase #5 $18.58   5.398%   $19.5829   $19.58   $19.60   $19.60  
  Purchase #6 $170.38   5.849%   $180.3455   $180.35   $180.35   $180.30  
  Purchase #7 $156.51   12.374%   $175.8765   $175.88   $175.90   $175.90  
  7 purchase totals = $580.20       $633.7972   $633.80   $633.85   $633.80