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 $82.88   8.761%   $90.1411   $90.14   $90.15   $90.10  
  Purchase #2 $159.60   12.804%   $180.0352   $180.04   $180.05   $180.00  
  Purchase #3 $15.90   8.720%   $17.2865   $17.29   $17.30   $17.30  
  Purchase #4 $146.59   10.821%   $162.4525   $162.45   $162.45   $162.50  
  Purchase #5 $32.75   6.230%   $34.7903   $34.79   $34.80   $34.80  
  Purchase #6 $112.44   13.818%   $127.977   $127.98   $128.00   $128.00  
  Purchase #7 $70.71   12.836%   $79.7863   $79.79   $79.80   $79.80  
  7 purchase totals = $620.87       $692.4689   $692.48   $692.55   $692.50