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 $5.74   7.544%   $6.173   $6.17   $6.15   $6.20  
  Purchase #2 $13.29   3.839%   $13.8002   $13.80   $13.80   $13.80  
  Purchase #3 $27.57   3.270%   $28.4715   $28.47   $28.45   $28.50  
  Purchase #4 $215.95   12.589%   $243.1359   $243.14   $243.15   $243.10  
  Purchase #5 $37.75   8.516%   $40.9648   $40.96   $40.95   $41.00  
  Purchase #6 $176.91   11.480%   $197.2193   $197.22   $197.20   $197.20  
  Purchase #7 $135.86   5.378%   $143.1666   $143.17   $143.15   $143.20  
  7 purchase totals = $613.07       $672.9313   $672.93   $672.85   $673.00