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 $83.89   7.304%   $90.0173   $90.02   $90.00   $90.00  
  Purchase #2 $61.25   5.152%   $64.4056   $64.41   $64.40   $64.40  
  Purchase #3 $123.84   12.260%   $139.0228   $139.02   $139.00   $139.00  
  Purchase #4 $197.86   13.806%   $225.1766   $225.18   $225.20   $225.20  
  Purchase #5 $147.43   9.754%   $161.8103   $161.81   $161.80   $161.80  
  Purchase #6 $12.99   4.464%   $13.5699   $13.57   $13.55   $13.60  
  Purchase #7 $12.25   11.799%   $13.6954   $13.70   $13.70   $13.70  
  7 purchase totals = $639.51       $707.6979   $707.71   $707.65   $707.70