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 $81.84   7.841%   $88.2571   $88.26   $88.25   $88.30  
  Purchase #2 $231.46   3.237%   $238.9524   $238.95   $238.95   $239.00  
  Purchase #3 $61.45   9.380%   $67.214   $67.21   $67.20   $67.20  
  Purchase #4 $88.53   8.549%   $96.0984   $96.10   $96.10   $96.10  
  Purchase #5 $189.90   3.802%   $197.12   $197.12   $197.10   $197.10  
  Purchase #6 $109.62   3.118%   $113.038   $113.04   $113.05   $113.00  
  Purchase #7 $90.66   10.437%   $100.1222   $100.12   $100.10   $100.10  
  7 purchase totals = $853.46       $900.8021   $900.80   $900.75   $900.80