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 $150.85   12.182%   $169.2265   $169.23   $169.25   $169.20  
  Purchase #2 $53.66   11.252%   $59.6978   $59.70   $59.70   $59.70  
  Purchase #3 $230.34   4.270%   $240.1755   $240.18   $240.20   $240.20  
  Purchase #4 $104.36   9.518%   $114.293   $114.29   $114.30   $114.30  
  Purchase #5 $149.47   13.590%   $169.783   $169.78   $169.80   $169.80  
  Purchase #6 $192.84   4.343%   $201.215   $201.22   $201.20   $201.20  
  Purchase #7 $23.37   6.288%   $24.8395   $24.84   $24.85   $24.80  
  7 purchase totals = $904.89       $979.2303   $979.24   $979.30   $979.20