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 $64.52   6.234%   $68.5422   $68.54   $68.55   $68.50  
  Purchase #2 $170.93   10.292%   $188.5221   $188.52   $188.50   $188.50  
  Purchase #3 $32.35   13.209%   $36.6231   $36.62   $36.60   $36.60  
  Purchase #4 $219.92   11.781%   $245.8288   $245.83   $245.85   $245.80  
  Purchase #5 $120.62   5.388%   $127.119   $127.12   $127.10   $127.10  
  Purchase #6 $96.52   6.672%   $102.9598   $102.96   $102.95   $103.00  
  Purchase #7 $196.55   12.692%   $221.4961   $221.50   $221.50   $221.50  
  7 purchase totals = $901.41       $991.0911   $991.09   $991.05   $991.00