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 $5.30   12.870%   $5.9821   $5.98   $6.00   $6.00  
  Purchase #2 $34.85   5.585%   $36.7964   $36.80   $36.80   $36.80  
  Purchase #3 $10.90   8.494%   $11.8258   $11.83   $11.85   $11.80  
  Purchase #4 $10.96   3.627%   $11.3575   $11.36   $11.35   $11.40  
  Purchase #5 $173.35   5.112%   $182.2117   $182.21   $182.20   $182.20  
  Purchase #6 $149.48   13.564%   $169.7555   $169.76   $169.75   $169.80  
  Purchase #7 $78.80   11.271%   $87.6815   $87.68   $87.70   $87.70  
  7 purchase totals = $463.64       $505.6105   $505.62   $505.65   $505.70