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 $223.71   13.844%   $254.6804   $254.68   $254.70   $254.70  
  Purchase #2 $162.85   7.416%   $174.927   $174.93   $174.95   $174.90  
  Purchase #3 $23.78   12.700%   $26.8001   $26.80   $26.80   $26.80  
  Purchase #4 $62.37   8.259%   $67.5211   $67.52   $67.50   $67.50  
  Purchase #5 $103.97   13.326%   $117.825   $117.83   $117.85   $117.80  
  Purchase #6 $52.65   11.333%   $58.6168   $58.62   $58.60   $58.60  
  Purchase #7 $135.75   4.604%   $141.9999   $142.00   $142.00   $142.00  
  7 purchase totals = $765.08       $842.3703   $842.38   $842.40   $842.30