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 $244.36   12.136%   $274.0155   $274.02   $274.00   $274.00  
  Purchase #2 $5.34   13.117%   $6.0404   $6.04   $6.05   $6.00  
  Purchase #3 $29.85   11.538%   $33.2941   $33.29   $33.30   $33.30  
  Purchase #4 $82.76   5.151%   $87.023   $87.02   $87.00   $87.00  
  Purchase #5 $5.33   10.375%   $5.883   $5.88   $5.90   $5.90  
  Purchase #6 $204.52   7.650%   $220.1658   $220.17   $220.15   $220.20  
  Purchase #7 $225.99   11.626%   $252.2636   $252.26   $252.25   $252.30  
  7 purchase totals = $798.15       $878.6854   $878.68   $878.65   $878.70