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 $60.74   12.174%   $68.1345   $68.13   $68.15   $68.10  
  Purchase #2 $74.33   11.633%   $82.9768   $82.98   $83.00   $83.00  
  Purchase #3 $237.34   8.365%   $257.1935   $257.19   $257.20   $257.20  
  Purchase #4 $193.91   10.506%   $214.2822   $214.28   $214.30   $214.30  
  Purchase #5 $241.53   12.444%   $271.586   $271.59   $271.60   $271.60  
  Purchase #6 $97.65   9.622%   $107.0459   $107.05   $107.05   $107.00  
  Purchase #7 $111.53   8.120%   $120.5862   $120.59   $120.60   $120.60  
  7 purchase totals = $1,017.03       $1,121.8051   $1,121.81   $1,121.90   $1,121.80