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 $235.53   12.416%   $264.7734   $264.77   $264.75   $264.80  
  Purchase #2 $12.31   8.479%   $13.3538   $13.35   $13.35   $13.40  
  Purchase #3 $54.47   7.282%   $58.4365   $58.44   $58.45   $58.40  
  Purchase #4 $126.67   7.298%   $135.9144   $135.91   $135.90   $135.90  
  Purchase #5 $248.88   10.235%   $274.3529   $274.35   $274.35   $274.40  
  Purchase #6 $89.48   8.327%   $96.931   $96.93   $96.95   $96.90  
  Purchase #7 $217.64   13.270%   $246.5208   $246.52   $246.50   $246.50  
  7 purchase totals = $984.98       $1,090.2828   $1,090.27   $1,090.25   $1,090.30