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 $213.63   13.171%   $241.7672   $241.77   $241.75   $241.80  
  Purchase #2 $109.75   7.399%   $117.8704   $117.87   $117.85   $117.90  
  Purchase #3 $58.34   9.256%   $63.74   $63.74   $63.75   $63.70  
  Purchase #4 $134.38   12.468%   $151.1345   $151.13   $151.15   $151.10  
  Purchase #5 $226.88   5.310%   $238.9273   $238.93   $238.95   $238.90  
  Purchase #6 $92.34   12.635%   $104.0072   $104.01   $104.00   $104.00  
  Purchase #7 $75.25   11.737%   $84.0821   $84.08   $84.10   $84.10  
  7 purchase totals = $910.57       $1,001.5287   $1,001.53   $1,001.55   $1,001.50