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 $8.74   4.372%   $9.1221   $9.12   $9.10   $9.10  
  Purchase #2 $15.72   7.243%   $16.8586   $16.86   $16.85   $16.90  
  Purchase #3 $38.41   5.800%   $40.6378   $40.64   $40.65   $40.60  
  Purchase #4 $220.99   9.500%   $241.9841   $241.98   $242.00   $242.00  
  Purchase #5 $123.38   5.215%   $129.8143   $129.81   $129.80   $129.80  
  Purchase #6 $247.83   9.434%   $271.2103   $271.21   $271.20   $271.20  
  Purchase #7 $91.78   11.828%   $102.6357   $102.64   $102.65   $102.60  
  7 purchase totals = $746.85       $812.2629   $812.26   $812.25   $812.20