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 $99.99   3.372%   $103.3617   $103.36   $103.35   $103.40  
  Purchase #2 $62.55   8.227%   $67.696   $67.70   $67.70   $67.70  
  Purchase #3 $129.37   9.345%   $141.4596   $141.46   $141.45   $141.50  
  Purchase #4 $238.74   13.861%   $271.8318   $271.83   $271.85   $271.80  
  Purchase #5 $11.30   11.525%   $12.6023   $12.60   $12.60   $12.60  
  Purchase #6 $26.95   9.742%   $29.5755   $29.58   $29.60   $29.60  
  Purchase #7 $15.28   8.288%   $16.5464   $16.55   $16.55   $16.50  
  7 purchase totals = $584.18       $643.0733   $643.08   $643.10   $643.10