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 $28.98   7.507%   $31.1555   $31.16   $31.15   $31.20  
  Purchase #2 $145.27   8.683%   $157.8838   $157.88   $157.90   $157.90  
  Purchase #3 $43.89   9.544%   $48.0789   $48.08   $48.10   $48.10  
  Purchase #4 $68.94   10.318%   $76.0532   $76.05   $76.05   $76.10  
  Purchase #5 $104.34   6.212%   $110.8216   $110.82   $110.80   $110.80  
  Purchase #6 $102.28   13.450%   $116.0367   $116.04   $116.05   $116.00  
  Purchase #7 $88.93   11.277%   $98.9586   $98.96   $98.95   $99.00  
  7 purchase totals = $582.63       $638.9883   $638.99   $639.00   $639.10