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 $21.72   9.484%   $23.7799   $23.78   $23.80   $23.80  
  Purchase #2 $40.65   7.752%   $43.8012   $43.80   $43.80   $43.80  
  Purchase #3 $247.60   11.296%   $275.5689   $275.57   $275.55   $275.60  
  Purchase #4 $97.29   13.369%   $110.2967   $110.30   $110.30   $110.30  
  Purchase #5 $20.26   13.871%   $23.0703   $23.07   $23.05   $23.10  
  Purchase #6 $51.41   9.510%   $56.2991   $56.30   $56.30   $56.30  
  Purchase #7 $123.49   4.728%   $129.3286   $129.33   $129.35   $129.30  
  7 purchase totals = $602.42       $662.1447   $662.15   $662.15   $662.20