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 $89.94   8.740%   $97.8008   $97.80   $97.80   $97.80  
  Purchase #2 $107.92   13.818%   $122.8324   $122.83   $122.85   $122.80  
  Purchase #3 $156.90   12.104%   $175.8912   $175.89   $175.90   $175.90  
  Purchase #4 $31.95   12.706%   $36.0096   $36.01   $36.00   $36.00  
  Purchase #5 $37.64   9.544%   $41.2324   $41.23   $41.25   $41.20  
  Purchase #6 $21.42   4.827%   $22.4539   $22.45   $22.45   $22.50  
  Purchase #7 $181.69   4.474%   $189.8188   $189.82   $189.80   $189.80  
  7 purchase totals = $627.46       $686.0391   $686.03   $686.05   $686.00