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 $17.37   6.294%   $18.4633   $18.46   $18.45   $18.50  
  Purchase #2 $143.91   4.178%   $149.9226   $149.92   $149.90   $149.90  
  Purchase #3 $29.44   9.441%   $32.2194   $32.22   $32.20   $32.20  
  Purchase #4 $78.57   10.720%   $86.9927   $86.99   $87.00   $87.00  
  Purchase #5 $45.65   13.160%   $51.6575   $51.66   $51.65   $51.70  
  Purchase #6 $58.68   4.693%   $61.4339   $61.43   $61.45   $61.40  
  Purchase #7 $238.88   3.733%   $247.7974   $247.80   $247.80   $247.80  
  7 purchase totals = $612.50       $648.4868   $648.48   $648.45   $648.50