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 $28.51   5.474%   $30.0706   $30.07   $30.05   $30.10  
  Purchase #2 $79.29   13.613%   $90.0837   $90.08   $90.10   $90.10  
  Purchase #3 $62.58   7.656%   $67.3711   $67.37   $67.35   $67.40  
  Purchase #4 $21.32   3.384%   $22.0415   $22.04   $22.05   $22.00  
  Purchase #5 $150.35   3.406%   $155.4709   $155.47   $155.45   $155.50  
  Purchase #6 $62.92   9.812%   $69.0937   $69.09   $69.10   $69.10  
  Purchase #7 $250.32   9.831%   $274.929   $274.93   $274.95   $274.90  
  7 purchase totals = $655.29       $709.0605   $709.05   $709.05   $709.10