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 $235.74   12.131%   $264.3376   $264.34   $264.35   $264.30  
  Purchase #2 $180.59   5.787%   $191.0407   $191.04   $191.05   $191.00  
  Purchase #3 $64.93   8.179%   $70.2406   $70.24   $70.25   $70.20  
  Purchase #4 $165.95   3.654%   $172.0138   $172.01   $172.00   $172.00  
  Purchase #5 $250.66   6.373%   $266.6346   $266.63   $266.65   $266.60  
  Purchase #6 $46.94   9.252%   $51.2829   $51.28   $51.30   $51.30  
  Purchase #7 $207.72   6.205%   $220.609   $220.61   $220.60   $220.60  
  7 purchase totals = $1,152.53       $1,236.1592   $1,236.15   $1,236.20   $1,236.00