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 $232.35   13.444%   $263.5871   $263.59   $263.60   $263.60  
  Purchase #2 $143.93   8.625%   $156.344   $156.34   $156.35   $156.30  
  Purchase #3 $89.27   6.562%   $95.1279   $95.13   $95.15   $95.10  
  Purchase #4 $126.44   13.712%   $143.7775   $143.78   $143.80   $143.80  
  Purchase #5 $112.97   11.562%   $126.0316   $126.03   $126.05   $126.00  
  Purchase #6 $68.51   7.711%   $73.7928   $73.79   $73.80   $73.80  
  Purchase #7 $129.71   4.129%   $135.0657   $135.07   $135.05   $135.10  
  7 purchase totals = $903.18       $993.7266   $993.73   $993.80   $993.70