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 $77.68   4.107%   $80.8703   $80.87   $80.85   $80.90  
  Purchase #2 $144.35   4.264%   $150.5051   $150.51   $150.50   $150.50  
  Purchase #3 $40.62   5.484%   $42.8476   $42.85   $42.85   $42.80  
  Purchase #4 $44.40   13.486%   $50.3878   $50.39   $50.40   $50.40  
  Purchase #5 $105.81   7.819%   $114.0833   $114.08   $114.10   $114.10  
  Purchase #6 $46.58   9.441%   $50.9776   $50.98   $51.00   $51.00  
  Purchase #7 $163.41   5.255%   $171.9972   $172.00   $172.00   $172.00  
  7 purchase totals = $622.85       $661.6689   $661.68   $661.70   $661.70