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 $107.62   12.402%   $120.967   $120.97   $120.95   $121.00  
  Purchase #2 $177.93   8.171%   $192.4687   $192.47   $192.45   $192.50  
  Purchase #3 $193.58   10.376%   $213.6659   $213.67   $213.65   $213.70  
  Purchase #4 $87.93   6.551%   $93.6903   $93.69   $93.70   $93.70  
  Purchase #5 $130.35   13.865%   $148.423   $148.42   $148.40   $148.40  
  Purchase #6 $69.69   10.366%   $76.9141   $76.91   $76.90   $76.90  
  Purchase #7 $144.48   7.640%   $155.5183   $155.52   $155.50   $155.50  
  7 purchase totals = $911.58       $1,001.6473   $1,001.65   $1,001.55   $1,001.70