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 $42.81   6.471%   $45.5802   $45.58   $45.60   $45.60  
  Purchase #2 $18.95   5.753%   $20.0402   $20.04   $20.05   $20.00  
  Purchase #3 $75.67   6.383%   $80.5   $80.50   $80.50   $80.50  
  Purchase #4 $189.32   12.643%   $213.2557   $213.26   $213.25   $213.30  
  Purchase #5 $122.85   4.260%   $128.0834   $128.08   $128.10   $128.10  
  Purchase #6 $25.44   8.391%   $27.5747   $27.57   $27.55   $27.60  
  Purchase #7 $119.90   10.718%   $132.7509   $132.75   $132.75   $132.80  
  7 purchase totals = $594.94       $647.7851   $647.78   $647.80   $647.90