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 $243.53   5.211%   $256.2203   $256.22   $256.20   $256.20  
  Purchase #2 $240.75   12.684%   $271.2867   $271.29   $271.30   $271.30  
  Purchase #3 $153.35   8.741%   $166.7543   $166.75   $166.75   $166.80  
  Purchase #4 $109.25   9.700%   $119.8473   $119.85   $119.85   $119.80  
  Purchase #5 $81.79   6.417%   $87.0385   $87.04   $87.05   $87.00  
  Purchase #6 $184.30   4.488%   $192.5714   $192.57   $192.55   $192.60  
  Purchase #7 $142.51   5.182%   $149.8949   $149.89   $149.90   $149.90  
  7 purchase totals = $1,155.48       $1,243.6134   $1,243.61   $1,243.60   $1,243.60