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 $20.45   8.341%   $22.1557   $22.16   $22.15   $22.20  
  Purchase #2 $77.98   13.187%   $88.2632   $88.26   $88.25   $88.30  
  Purchase #3 $13.76   10.731%   $15.2366   $15.24   $15.25   $15.20  
  Purchase #4 $238.56   12.706%   $268.8714   $268.87   $268.85   $268.90  
  Purchase #5 $207.75   10.194%   $228.928   $228.93   $228.95   $228.90  
  Purchase #6 $14.45   10.611%   $15.9833   $15.98   $16.00   $16.00  
  Purchase #7 $70.88   4.638%   $74.1674   $74.17   $74.15   $74.20  
  7 purchase totals = $643.83       $713.6056   $713.61   $713.60   $713.70