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 $240.82   12.803%   $271.6522   $271.65   $271.65   $271.70  
  Purchase #2 $135.51   6.512%   $144.3344   $144.33   $144.35   $144.30  
  Purchase #3 $69.40   8.846%   $75.5391   $75.54   $75.55   $75.50  
  Purchase #4 $196.46   11.166%   $218.3967   $218.40   $218.40   $218.40  
  Purchase #5 $197.38   11.608%   $220.2919   $220.29   $220.30   $220.30  
  Purchase #6 $200.28   3.114%   $206.5167   $206.52   $206.50   $206.50  
  Purchase #7 $103.64   11.327%   $115.3793   $115.38   $115.40   $115.40  
  7 purchase totals = $1,143.49       $1,252.1103   $1,252.11   $1,252.15   $1,252.10