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 $6.83   13.193%   $7.7311   $7.73   $7.75   $7.70  
  Purchase #2 $81.31   3.414%   $84.0859   $84.09   $84.10   $84.10  
  Purchase #3 $220.78   9.198%   $241.0873   $241.09   $241.10   $241.10  
  Purchase #4 $177.43   9.812%   $194.8394   $194.84   $194.85   $194.80  
  Purchase #5 $186.41   8.542%   $202.3331   $202.33   $202.35   $202.30  
  Purchase #6 $8.84   7.518%   $9.5046   $9.50   $9.50   $9.50  
  Purchase #7 $88.98   12.510%   $100.1114   $100.11   $100.10   $100.10  
  7 purchase totals = $770.58       $839.6928   $839.69   $839.75   $839.60