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 $241.54   13.580%   $274.3411   $274.34   $274.35   $274.30  
  Purchase #2 $204.92   13.306%   $232.1867   $232.19   $232.20   $232.20  
  Purchase #3 $49.80   8.316%   $53.9414   $53.94   $53.95   $53.90  
  Purchase #4 $142.97   9.419%   $156.4363   $156.44   $156.45   $156.40  
  Purchase #5 $73.77   7.796%   $79.5211   $79.52   $79.50   $79.50  
  Purchase #6 $114.92   10.817%   $127.3509   $127.35   $127.35   $127.40  
  Purchase #7 $16.88   8.306%   $18.2821   $18.28   $18.30   $18.30  
  7 purchase totals = $844.80       $942.0596   $942.06   $942.10   $942.00