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 $11.58   10.243%   $12.7661   $12.77   $12.75   $12.80  
  Purchase #2 $226.56   6.736%   $241.8211   $241.82   $241.80   $241.80  
  Purchase #3 $68.90   3.391%   $71.2364   $71.24   $71.25   $71.20  
  Purchase #4 $176.94   8.113%   $191.2951   $191.30   $191.30   $191.30  
  Purchase #5 $163.40   9.252%   $178.5178   $178.52   $178.50   $178.50  
  Purchase #6 $22.52   6.260%   $23.9298   $23.93   $23.95   $23.90  
  Purchase #7 $125.29   8.327%   $135.7229   $135.72   $135.70   $135.70  
  7 purchase totals = $795.19       $855.2892   $855.30   $855.25   $855.20