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 $28.45   8.560%   $30.8853   $30.89   $30.90   $30.90  
  Purchase #2 $193.74   13.467%   $219.831   $219.83   $219.85   $219.80  
  Purchase #3 $128.45   8.257%   $139.0561   $139.06   $139.05   $139.10  
  Purchase #4 $119.60   3.452%   $123.7286   $123.73   $123.75   $123.70  
  Purchase #5 $7.54   6.352%   $8.0189   $8.02   $8.00   $8.00  
  Purchase #6 $35.86   7.220%   $38.4491   $38.45   $38.45   $38.40  
  Purchase #7 $89.98   10.449%   $99.382   $99.38   $99.40   $99.40  
  7 purchase totals = $603.62       $659.3510   $659.36   $659.40   $659.30