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 $9.25   5.804%   $9.7869   $9.79   $9.80   $9.80  
  Purchase #2 $16.94   8.483%   $18.377   $18.38   $18.40   $18.40  
  Purchase #3 $178.50   12.835%   $201.4105   $201.41   $201.40   $201.40  
  Purchase #4 $28.26   10.705%   $31.2852   $31.29   $31.30   $31.30  
  Purchase #5 $25.36   11.517%   $28.2807   $28.28   $28.30   $28.30  
  Purchase #6 $159.99   3.209%   $165.1241   $165.12   $165.10   $165.10  
  Purchase #7 $203.87   12.337%   $229.0214   $229.02   $229.00   $229.00  
  7 purchase totals = $622.17       $683.2858   $683.29   $683.30   $683.30