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 $169.38   7.303%   $181.7498   $181.75   $181.75   $181.70  
  Purchase #2 $120.39   7.808%   $129.7901   $129.79   $129.80   $129.80  
  Purchase #3 $6.69   11.639%   $7.4686   $7.47   $7.45   $7.50  
  Purchase #4 $147.54   6.246%   $156.7553   $156.76   $156.75   $156.80  
  Purchase #5 $18.39   5.588%   $19.4176   $19.42   $19.40   $19.40  
  Purchase #6 $25.86   11.134%   $28.7393   $28.74   $28.75   $28.70  
  Purchase #7 $111.28   6.123%   $118.0937   $118.09   $118.10   $118.10  
  7 purchase totals = $599.53       $642.0144   $642.02   $642.00   $642.00