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 $244.55   12.314%   $274.6639   $274.66   $274.65   $274.70  
  Purchase #2 $35.86   7.567%   $38.5735   $38.57   $38.55   $38.60  
  Purchase #3 $98.29   11.438%   $109.5324   $109.53   $109.55   $109.50  
  Purchase #4 $33.95   3.125%   $35.0109   $35.01   $35.00   $35.00  
  Purchase #5 $148.51   8.186%   $160.667   $160.67   $160.65   $160.70  
  Purchase #6 $155.51   9.169%   $169.7687   $169.77   $169.75   $169.80  
  Purchase #7 $65.77   10.671%   $72.7883   $72.79   $72.80   $72.80  
  7 purchase totals = $782.44       $861.0047   $861.00   $860.95   $861.10