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 $38.74   3.187%   $39.9746   $39.97   $39.95   $40.00  
  Purchase #2 $124.25   10.341%   $137.0987   $137.10   $137.10   $137.10  
  Purchase #3 $138.96   10.388%   $153.3952   $153.40   $153.40   $153.40  
  Purchase #4 $179.73   13.426%   $203.8605   $203.86   $203.85   $203.90  
  Purchase #5 $131.44   9.748%   $144.2528   $144.25   $144.25   $144.30  
  Purchase #6 $67.41   10.630%   $74.5757   $74.58   $74.60   $74.60  
  Purchase #7 $166.58   5.549%   $175.8235   $175.82   $175.80   $175.80  
  7 purchase totals = $847.11       $928.9810   $928.98   $928.95   $929.10