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 $20.92   4.319%   $21.8235   $21.82   $21.80   $21.80  
  Purchase #2 $31.50   8.330%   $34.124   $34.12   $34.10   $34.10  
  Purchase #3 $180.84   10.224%   $199.3291   $199.33   $199.35   $199.30  
  Purchase #4 $250.75   13.233%   $283.9317   $283.93   $283.95   $283.90  
  Purchase #5 $106.65   3.850%   $110.756   $110.76   $110.75   $110.80  
  Purchase #6 $9.65   12.215%   $10.8287   $10.83   $10.85   $10.80  
  Purchase #7 $137.86   6.665%   $147.0484   $147.05   $147.05   $147.00  
  7 purchase totals = $738.17       $807.8414   $807.84   $807.85   $807.70