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 $249.93   13.236%   $283.0107   $283.01   $283.00   $283.00  
  Purchase #2 $175.95   13.872%   $200.3578   $200.36   $200.35   $200.40  
  Purchase #3 $43.38   9.585%   $47.538   $47.54   $47.55   $47.50  
  Purchase #4 $55.62   7.549%   $59.8188   $59.82   $59.80   $59.80  
  Purchase #5 $213.81   12.557%   $240.6581   $240.66   $240.65   $240.70  
  Purchase #6 $189.29   3.156%   $195.264   $195.26   $195.25   $195.30  
  Purchase #7 $187.88   8.645%   $204.1222   $204.12   $204.10   $204.10  
  7 purchase totals = $1,115.86       $1,230.7696   $1,230.77   $1,230.70   $1,230.80