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 $20.43   4.607%   $21.3712   $21.37   $21.35   $21.40  
  Purchase #2 $39.93   4.247%   $41.6258   $41.63   $41.65   $41.60  
  Purchase #3 $102.66   7.276%   $110.1295   $110.13   $110.15   $110.10  
  Purchase #4 $223.98   4.268%   $233.5395   $233.54   $233.55   $233.50  
  Purchase #5 $110.59   5.653%   $116.8417   $116.84   $116.85   $116.80  
  Purchase #6 $112.74   3.320%   $116.483   $116.48   $116.50   $116.50  
  Purchase #7 $143.73   5.194%   $151.1953   $151.20   $151.20   $151.20  
  7 purchase totals = $754.06       $791.1860   $791.19   $791.25   $791.10