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 $236.53   9.299%   $258.5249   $258.52   $258.50   $258.50  
  Purchase #2 $192.86   11.632%   $215.2935   $215.29   $215.30   $215.30  
  Purchase #3 $239.79   6.805%   $256.1077   $256.11   $256.10   $256.10  
  Purchase #4 $180.86   11.667%   $201.9609   $201.96   $201.95   $202.00  
  Purchase #5 $219.44   10.211%   $241.847   $241.85   $241.85   $241.80  
  Purchase #6 $182.41   10.489%   $201.543   $201.54   $201.55   $201.50  
  Purchase #7 $113.93   12.654%   $128.3467   $128.35   $128.35   $128.30  
  7 purchase totals = $1,365.82       $1,503.6237   $1,503.62   $1,503.60   $1,503.50