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 $172.70   4.147%   $179.8619   $179.86   $179.85   $179.90  
  Purchase #2 $169.65   13.211%   $192.0625   $192.06   $192.05   $192.10  
  Purchase #3 $140.82   6.827%   $150.4338   $150.43   $150.45   $150.40  
  Purchase #4 $116.44   11.562%   $129.9028   $129.90   $129.90   $129.90  
  Purchase #5 $217.61   7.804%   $234.5923   $234.59   $234.60   $234.60  
  Purchase #6 $25.74   9.501%   $28.1856   $28.19   $28.20   $28.20  
  Purchase #7 $68.82   7.344%   $73.8741   $73.87   $73.85   $73.90  
  7 purchase totals = $911.78       $988.9130   $988.90   $988.90   $989.00