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 $18.50   12.405%   $20.7949   $20.79   $20.80   $20.80  
  Purchase #2 $67.35   12.760%   $75.9439   $75.94   $75.95   $75.90  
  Purchase #3 $128.79   7.743%   $138.7622   $138.76   $138.75   $138.80  
  Purchase #4 $6.68   11.663%   $7.4591   $7.46   $7.45   $7.50  
  Purchase #5 $202.78   13.153%   $229.4517   $229.45   $229.45   $229.50  
  Purchase #6 $177.38   3.601%   $183.7675   $183.77   $183.75   $183.80  
  Purchase #7 $158.88   8.572%   $172.4992   $172.50   $172.50   $172.50  
  7 purchase totals = $760.36       $828.6785   $828.67   $828.65   $828.80