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 $63.94   4.657%   $66.9177   $66.92   $66.90   $66.90  
  Purchase #2 $143.33   8.260%   $155.1691   $155.17   $155.15   $155.20  
  Purchase #3 $88.91   11.382%   $99.0297   $99.03   $99.05   $99.00  
  Purchase #4 $238.57   13.675%   $271.1944   $271.19   $271.20   $271.20  
  Purchase #5 $27.38   8.455%   $29.695   $29.70   $29.70   $29.70  
  Purchase #6 $85.60   12.399%   $96.2135   $96.21   $96.20   $96.20  
  Purchase #7 $50.56   9.628%   $55.4279   $55.43   $55.45   $55.40  
  7 purchase totals = $698.29       $773.6473   $773.65   $773.65   $773.60