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 $21.73   6.783%   $23.2039   $23.20   $23.20   $23.20  
  Purchase #2 $120.45   5.171%   $126.6785   $126.68   $126.70   $126.70  
  Purchase #3 $122.83   8.113%   $132.7952   $132.80   $132.80   $132.80  
  Purchase #4 $147.35   9.681%   $161.615   $161.62   $161.60   $161.60  
  Purchase #5 $83.98   8.433%   $91.062   $91.06   $91.05   $91.10  
  Purchase #6 $218.84   11.508%   $244.0241   $244.02   $244.00   $244.00  
  Purchase #7 $171.62   7.388%   $184.2993   $184.30   $184.30   $184.30  
  7 purchase totals = $886.80       $963.6780   $963.68   $963.65   $963.70