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 $120.80   6.442%   $128.5819   $128.58   $128.60   $128.60  
  Purchase #2 $34.78   10.678%   $38.4938   $38.49   $38.50   $38.50  
  Purchase #3 $43.99   13.814%   $50.0668   $50.07   $50.05   $50.10  
  Purchase #4 $122.54   6.386%   $130.3654   $130.37   $130.35   $130.40  
  Purchase #5 $138.93   3.170%   $143.3341   $143.33   $143.35   $143.30  
  Purchase #6 $112.51   4.251%   $117.2928   $117.29   $117.30   $117.30  
  Purchase #7 $181.79   4.209%   $189.4415   $189.44   $189.45   $189.40  
  7 purchase totals = $755.34       $797.5763   $797.57   $797.60   $797.60